Discourse Structu res for Text Generation 
William C. Mann 
USC/Intorrnation Sciences Institute 
4676 Admiralty Way 
Marina del Rey, CA 90292-6695 
A bst ract 
Text generation programs need to be designed around a 
theory of text organization. This paper introduces Rhetorical 
Structure Theory, a theory of text structure in which each region 
of text has a central nuclear part and a number of satellites 
related to it. A natural text is analyzed as an example, the 
mechanisms of the theory are identified, and their formalization is 
discussed. In a comparison, Rhetorical Structure Theory is found 
to be more comprehensive and more informative about text 
function than the text organization parts of previous text 
generation systems. 
1, The Text Organization Problem 
Text generation is already established as a research area 
within computational linguistics. Although so far there have been 
only a few research computer programs that can generate text in a 
technically interesting way, text generation is recognized as 
having problems and accomplishments that are distinct from 
those of the rest of computational linguistics. Text generation 
involves creation of multisentential text without any direct use of 
people's linguistic skills; it is not computer-aided text creation. 
Text planning is a major activity within text generation, one 
that strongly influences the effectiveness of generated text. 
Among the things that have been taken to be part of text planning, 
this paper focuses on just one: text oreanization. People 
commonly recognize that well.written text is organized, and that it 
succeeds partly by exhibiting its organization to the reader. 
Computer generated text must be organized. To create 
This research was supported by the Air Force Office of Scientific 
Research contract No. F49620-79-C-0181. The views and 
conclusions contained in this document are those of the author 
and should not be interpreted as necessarily representing the 
official policies or endorsements, either expressed or implied, of 
the Air Force Office of Scientific Research of the U.S. 
Government. 
text generators, we must first have a suitable theory of text 
organization. In order to be most useful in computational 
linguistics, we want a theory of text organization to have these 
attributes: 
1. comprehensiveness: applicable to every kind of 
text; 
2. functionality: informative in terms of how text 
achieves its effects for the writer; 
3. scale insensitivity: applicable to every size of text, 
and capable of describing all of the various sized units 
of text organization that occur; 
4. definiteness: susceptible to formalization and 
programming; 
5. generativity: capable of use in text construction as 
well as text description. 
Unfortunately, no such theory exists. Our approach to creating 
such a theory is described below, and then compared with 
previous work on text generation in Section 3. 
2. Rhetorical Structure Theory 
Creating a comprehensive theory of text organization is 
necessarily a very complex effort. In order to limit the immediate 
complexity of the task we have concentrated first on creating a 
descriotiv~ theory, one which fits naturally occurring text. In the 
future the descriptive theory will be augmented in order to create a 
constructive theory, one which can be implemented for text 
generation. The term Rhetorical Structure Theory (RST) refers to 
the combination of the descriptive and constructive parts. 
An organized text is one which is composed of discernible 
parts, with the parts arranged in a particular way and connected 
together to form a whole. Therefore a theory of text organization 
must tell at least: 
1. What kinds of parts are there? 
2. How can parts be arranged? 
3. How can parts be connected together to form a whole text? 
367 
In RST we specify all of these jointly, identifying the organizational 
resources available to the writer. 
2.1. Descriptive Rhetorical Structure Theory 1 
What are the organizational resources available to the 
writer?. Here we present the mechanisms and character of 
rhetorical structure theory by showing how we have applied it to a 
particular natural text. As each new construct is introduced in the 
example, its abstract content is described. 
Our illustrative text is shown in Figure 2.1.23 In the figure, 
we have divided the running text into numbered clause-like units. 4 
At the highest level, the text is a request addressed to CCC 
members to vote against making the nuclear freeze initiative (NFI) 
one of the issues about which CCC actively lobbies and promotes 
a position. The structure of the text at this level consists of two 
parts: the request (clause 13) and the material put forth to support 
the request (clauses 1 through 12), 
2.1.1. The Request Schema --. 1-12; 13 
To represent the highest level of structure, we use the 
Request schema shown in Figure 2-2. The Request schema is 
one of about 25 schemas in the current version of RST. 
Each schema indicates how a particular unit of text 
structure is decomposed into other units. Such units are called 
spans. Spans are further differentiated into text spans and 
conceptual spans, text spans denoting the portion of explicit 
text being described, and conceptual spans denoting clusters of 
propositions concerning the subject matter (and sometimes the 
process of expressing it) being expressed by the text span. 
1The descriptive portion of rhetorical structure theory has been developed over 
the pest two years by Sandra Thoml:~son and me, with major contributions by 
Christian Matthiassen and Barbara Fox. They have also given helpful reactions to 
a previous draft of this paper. 
2Quoted (with permission) from The InsidQr, California Common Cause state 
newsletter, 2.1, July 1982. 
3We expect the generation of this sort of text to eventually become very 
impo~Rant in Artificial Intelligence, because systems will have to establish the 
acceptability of their conclusions on heuristic grounds. AI systems will have to 
establish their credibility by arguing for it in English. 
4Although we have not used technically-defined clauses as units, the character 
of the theory is not affected. The decision concerning what will be the finast-grain 
unit of description is rather arbitrary; here it is set by a preliminary syntax.oriented 
manuel process which identifies low-level, relatively independent units to use in 
the discourse analysis. One reason for picking such units is that we intend to build 
a text generator in which most smaller units are organized by a programmed 
grammar \[Mann & Matthieasen 3.\]. 
1. I don't believe that endorsing the Nuclear Freeze 
Initiative is the right step for California CC. 
2. Tempting as it may be, 
3. we shouldn't embrace every popular issue that comes 
along. 
4. When we do so 
we use precious, limited resources where other 
players with superior resources are already doing an 
adequate job. 
6. Rather, I think we will be stronger and more effective 
7. if we stick to those issues of governmental structure 
and process, broadly defined, that have formed the 
core of our agenda for years. 
8. Open government, campaign finance reform, and 
fighting the influence of special interests and big 
money, these are our kinds of issues. 
9. (New paragraph) Let's be clear: 
10. I personally favor the initiative and ardently support 
disarmament negotiations to reduce the risk of war. 
11. But I don't think endorsing a specific nuclear freeze 
proposal is appropriate fol: CCC. 
12. We should limit our involvement in defense and 
weaponry to matters of process, such as exposing the 
weapons industry's influence on the political process. 
13. Therefore, I urge you to vote against a CCC 
endorsement of the nuclear freeze initiative. 
(signed) Michael Asimow, California Common Cause 
Vice-Chair and UCLA Law Professor 
Figure 2.1 : A text which urges an action 
Each schema diagram has a vertical line indicating that 
one particular part is nuclear. The nuclear part is the one whose 
function most nearly represents the function of the text span 
analyzed in the structure by using the schema. In the example, 
clause 13 ("Therefore, I urge you to vote against a CCC 
endorsement of the nuclear freeze initiative.") is nuclear. It is a 
request. If it could plausibly have been successful by itself, 
something like clause 13 (without "Therefore") might have been 
used instead of the entire text. However, in this case, the writer 
did not expect that much to be enough, so some additional 
support was added. 
368 
Request 
~/~~e~ablement 
Evidence 
Figure 2-2: The Request and Evidence schemas 
The support, clauses 1 through 12, plays a satellite role in 
this application of the Request schema. Here, as in most cases, 
satellite text is used to make it more likely that the nuclear text will 
succeed. In this example, the writer is arguing that the requested 
action is right for the organization. 
In Figure 2-2 the nucleus is connected to each satellite by 
a relation. In the text clause 13 is related to clauses 1 through 12 
by a motivation relation. Clauses 1 through 12 are being used to 
motivate the reader to perform the action put forth in clause 13. 
The relations relate the conceptual span of a nucleus with 
the conceptual span of a satellite. Since, in s text structure, each 
conceptual span corresponds to a text span, the relations may be 
more loosely spoken of as relating text spans as well. 
The ReQuest schema also contains an eneblement 
relation. Text in an "enablement" relation to the nucleus conveys 
information (such as a password or telephone number) that makes 
the reader able to perform the requested action. In this example 
the option is not taken of having a satellite related to the nucleus 
by an "enablement" relation. 
One or more schemas may be instsntiated in a text. The 
pattern of instantiation of schemas in a text is called a text 
structure. So, for our example text, one part of its text structure 
says that the text span of the whole text corresponds to an 
instance of the Request schema, and that in that instance clause 
13 is the text span corresponding to the schema nucleus and 
clauses 1 through 12 are the text span corresponding to a satellite 
related to the nucleus by a "motivation" relation. 
In any instance of a schema in a text structure, the nucleus 
must be present, but all satellites are optional. We s do not 
instantiate a schema unless it shows some decomposition of its 
text span, so at least one of the satellites must be present. Any of 
the relations of a schema may be instantiated indefinitely many 
times, producing indefinitely many satellites. 
5Here and below, the knowledgeable person using RST to describe a text. 
The schemas do not restrict the order of textual elements. 
There is a usual order, the one which is most frequent when the 
schema is used to describe a large text span; schemas are drawn 
with this order in the figures describing them apart from their 
instantiation in text structure. However, any order is allowed. 
2.1.2. The Evidence Schema ... 1; 2-8; 9-12 
At the second level of decomposition each of the two text 
spans of the first level must be accounted for. The final text span, 
clause 13, is a single unit. For more detailed description a suitable 
grammar (and other companion theories) could be employed at 
this point. 
The initial span, clauses 1 through 12, consists of three 
parts: an assertion of a particular claim, clause 1, and two 
arguments supporting that claim, clauses 2 through 8 and 9 
through 12. The claim says that it would not be right for CCC to 
endorse the nuclear freeze initiative (NFI). The first argument is 
about how to allocate CCC's resources, and the second argument 
is about the categories of issues that CCC is best able to address. 
To represent this argument structure we use the Evidence 
schema, shown in Figure 2-2. Conceptual spans in an evidence 
relation stand as evidence that the conceptual span of the nucleus 
is correct. 
Note that the Evidence schema could not have been 
instantiated in place of the Request schema as the most 
comprehensive structure of the text, because clause 13 urges an 
action rather than supporting credibility. The "motivation" 
relation and the "evidence" relation restrict the nucleus in 
different ways, and thus provide application conditions on the 
schemas. The relations are perhaps the most restrictive source of 
conditions on how the schemas may apply. In addition, there are 
other application conventions for the schema, described in 
Section 2.2.3. 
The top two levels of structure of the text, the portion 
analyzed so far, are shown in Figure 2-3. The entire structure is 
shown in Figure 2-5. 
369 
Rcqunl 
Ev~enct 
1 2 3 4 5 6 7 8 9 10 tt 12 13 
Figu re 2-3: The upper structure of the CCC text 
At each level of structure it is possible to trace down the 
chain of nuclei to find a single clause which is representative of 
the entire level. Thus the representative of the whole text is clause 
13 (about voting), the representative of the first argument is clause 
6 (about being stronger and more effective), and the 
representative of the second argument is clause 12 (about limiting 
involvement to process issues). 
2.1.3. The Thesis/Antithesis Schema --- 2-5; 6-8 
The first argument is organized contrastively, in terms of 
one collection of ideas which the writer does not identify with, and 
a second collection of ideas which the writer does identify with. 
The first collection involves choosing issues on the basis of their 
popularity, a method which the writer opposes. The second 
collection concerns choosing issues of the kinds which have been 
successfully approached in the past, a method which the writer 
supports. 
To account for this pattern we use the Thesis/Antithesis 
schema shown in Figure 2.4. The ideas the writer is rejecting, 
clauses 2 through 5, are connected to the nucleus (clauses 6 
through 8) by a Thesis/Antithesis relation, which requires that 
the respective sections be in contrast and that the writer identify 
or not identify with them appropriately. 
Notice that in our instantiations of the Evidence schema 
and the Thesis/Antithesis schema, the roles of the nuclei relative 
to the satellites are similar: Under favorable conditions, the 
satellites would not be needed, but under the conditions as the 
author conceives them, the satellites increase the likelihood that 
the nucleus will succeed. The assertion of clause 1 is more likely 
to succeed because the evidence is present; the antithesis idea is 
made clearer and more appealing by rejecting the competing 
thesis idea. The Evidence schema is different from the 
Thesis/Antithesis schema because evidence and theses provide 
different kinds of support for assertions. 
2.1.4. The Evidence Schema --- 2-3; 4-5 6 
In RST, schemes are recursive. So, the Evidence schema 
can be instantiated to account for a text span identified by any 
schema, including the Evidence schema itself. This text illustrates 
this recursive character only twice, but mutual inclusion of 
schemas is actually used very frequently in general. It is the 
recursiveness of schemas which makes RST applicable at a wide 
range of scales, and which also allows it to describe structural 
units at a full range of sizes within a text. 7 
Clauses 2 and 3 make a statement about popular causes 
(centrally, that "we shouldn't embrace every popular issue that 
comes along"). Clauses 4 and 5 provide evidence that we 
shouldn't embrace them, in the form of an argument about 
effective use of resources. 
The Evidence schema shown in Figure 2.2 has thus been 
used again, this time with only one satellite. 
2.1.5. The Concessive Schema --- 2; 3 
Clause 2 suggests that embracing every popular issue is 
tempting (and thus both attractive and defective). The 
attractiveness of the move is acknowledged in the notion of a 
popular issue. Clause 3 identifies the defect: resources are used 
badly. 
The corresponding schema is the Concessive schema, 
shown in Figure 2-4. The concession relation relates the 
conceded conceptual span to the conceptual span which the 
writer is emphasizing. The "concession" relation differs from the 
"thesis/antithesis" relation in acknowledging the conceptual 
6Except for single-clause text spans, the structure of the text is presented 
depth-first, left to right, and shown in Figuro 2-5. 
7This contrasts with some approaches to text structure which do not provide 
structure between the whole-text level and the clause level. Stodes, 
problem-solution texts, advertisements, and interactive discourse have been 
analyzed in that way, 
370 
Thes Ls /A nlithesis Concessive 
inform 
b.7,ou  
Justify Conditional 
Figure 2-4: Five other schemas 
IIII I IIIIIIIiiiii IIIIIllllllllllll I Illlllllll i iiIll 
R~eff 
~blll 
1 TheSu/AnrltheJiJ 
Co~m~ Cmm',m~,m/ 
.... 7,.-" ~ ,o7---~ I 
Info~ 
Co~d,m~l 
2 3 4 5 6 ? 
Justly. 
ImD~at~ C(WCeUI~ 
77Wu/Amlalb~J 
10 
|~l|l/Int I|~ / 
11 12 
3 4 S 6 ? g 
V 
g 10 11 12 
Figu re 2.5: The full rhetorical structure of the CCC text 
'7 
371 
span of the satellite. The strategy for using a concessive is to 
acknowledge some potential detraction or refutation of the point 
to be made, By accepting it, it is seen as not contradictory with 
other beliefs held in the same context, and thus not a real 
refutation for the main point. 
Concessive structures are abundant in text that argues 
points which the writer sees as unpopular or in conflict with the 
audience's strongly held beliefs. In this text (which has two 
Concessive structures), we can infer that the writer believes that 
his audience strongly supports the NFI. 
2.1.6. The Conditional Schema --- 4; 5 
Clauses 4 and 5 present a consequence of embracing 
"every popular issue that comes along." Clause 4 ("when we do 
so") presents a condition, and clause 5 a result (use of resources) 
that occurs specifically under that condition. TO express this, we 
use the Conditional schema shown in Figure 2-4. The condition 
is related to the nuclear part by a condition relation, which 
carries the appropriate application restrictions to maintain the 
conditionality of the schema. 
2.1.7. The Inform Schema --- 6-7; 8 
The central assertion of the first argument, in clauses 6 
through 8, is that CCC can be stronger and more effective under 
the condition that it sticks to certain kinds of issues (implicitly 
excluding NFI). This assertion is then elaborated by exemplifying 
the kinds of issues meant. 
This presentation is described by applying the Inform 
schema shown in Figure 2-4. The central assertion is nuclear, and 
the detailed identification of kinds of issues is related to it by an 
elaboration relation. The option of having a span in the 
instantiation of the Inform schema related to the nucleus by a 
background relation is not taken. 
This text is anomalous among expository texts in not 
making much use of the Inform schema. 8 It is widely used, in part 
because it carries the "elaboration" relation. The "elaboration" 
relation is particularly versatile. It supplements the nuclear 
statement with various kinds of detail, including relationships of: 
1. sat:member 
2. abstraction:instance 
3. whole:part 
4. process:step 
5. object:attribute 
2.1.8. The Conditional Schema --- 6; 7 
This second use of the Conditional schema is unusual 
principally because the condition (clause 7) is expressed after the 
.consequence (clause 6). This may make the consequence more 
prominent or make it seem less uncertain. 
2.1.9. The JustifySchema --- 9; 10-12 
The writer has argued his case to a conclusion, and now 
wants to argue for this unpopular conclusion again. To gain 
acceptance for this tactic, and perhaps to show that a second 
argument is beginning, he says "Let's be clear." This is an 
instance of the Justify schema, shown in Figure 2-4. Here the 
satellite is attempting to make acceptable the act of exoressinq the 
nuclear conceptual span. 
2.1.10. The Concessive Schema -.- 10; 1 1-12 
The writer again employs the concessive schema, this time 
to show that favoring the NFI is consistent with voting against 
having CCC endorse it. In clause 10, the writer concedes that he 
personally favors the NFI. 
2.1.1 1. The Thesis/Antithesis Schema -.- 1 1 ; 12 
The writer states his position by contrasting two actions: 
CCC endorsing the NFI, which he does not approve, and CCC 
acting on matters of process, which he does approve. 
2.2. The Mechanisms of Descriptive RST 
In the preceding example we have seen how rhetorical 
schemas can be used to describe text. This section describes the 
three basic mechanisms of descriptive RST which have been 
exemplified above: 
1. Schemas 
2. Relation Definitions 
3. Schema Application Conventions 
2.2.1. Schemas 
A schema is defined entirely by identifying the set of 
relations which can relate a satellite to the nucleus. 
2.2.2. Relation Definitions 
A relation is defined by specifying three kinds of 
information: 
1. A characterization of the nucleus, 
2. A characterization of the satellite, 
3. A characterization of what sorts of interactions 
between the conceptual span of the nucleus and the 
conceptual span of the satellite must be plausible, s 
8It is also anomalous in another way: the widely used pattern of presenting a 
problem and its solution does not occur in this text. 9All of these characterizations must be made propedy relative to the writer's viewpoint and knowledge. 
372 
In addition, the relations are heavily involved in implicit 
communication; if this aspect is to be described, the relation 
definition must be extended accordingly. This aspect is outside of 
the scope of this paper but is discussed at length in \[Mann & 
Thompson 83\]. 
So, for example, to define the "motivation" relation, we 
would include at least the following material: 
1. The nucleus is an action performable but not yet 
performed by the reader. 
2. The satellite describes the action, the situation in 
which the action takes place, or the result of the 
action, in ways which help the reader to associate 
value assessments with the action. 
3. The value assessments are positive (to lead the reader 
to want to perform the action). 
2.2.3. Schema Application Conventions 
Most of the schema application conventions have already 
been mentioned: 
1. One schema is instantiated to describe the entire text. 
2. Schemas are instantiated to describe the text spans 
produced in instantiating other schemas. 
3. The schemas do not constrain the order of nucleus or 
satellites in the text span in which the schema is 
instantiated. 
4. All satellites are optional. 
5. At least one satellite must occur. 
6. A relation which is part of a schema may be 
instantiated indefinitely many times in the instantiation 
of that schema. 
7. The nucleus and satellites do not necessarily 
correspond to a single uninterrupted text span. 
Of course, there are strong patterns in the use of schemas 
in text: relations tend to be used just once, nucleus and satellites 
tend to occur in certain orders, and schemas tend to be used On 
uninterrupted spans of text. 
The theory currently contains about 25 schemas and 30 
relations. 1° We have applied it to a diverse collection of 
approximately 100 short natural texts, including administrative 
memos, advertisements, personal letters, newspaper articles, and 
magazine articles. These analyses have identified the usual 
patterns of schema use, along with many interesting exceptions. 
The theory is currently informal. Applying it requires 
making judgments about the applicability of the relations, e.g., 
what counts as evidence or as an attempt to motivate or justify 
some action. These are complex judgments, not easily formalized. 
10In this paper we do not separate the theow into framework and schemas, 
zdthough for other purposes there is a clear advantage and possibility of doing so. 
In its informal form the theory is still quite useful as a part of a 
linguistic approach to discourse. We do not expect to formalize it 
before going on to create a constructive theory. (Of course, since 
the constructive theory specifies text construction rather than 
describing natural texts, it need not depend on human judgements 
in the same way that the descriptive theory does.) 
2.3. Assessing Descriptive RST 
The most basic requirement on descriptive RST is that it be 
capable of describing the discernible organizational properties of 
natural texts, i.e., that it be a theory of discourse organization. 
The example above and our analyses of other texts have satisfied 
us that this is the case. 11 
tn addition, we want the theory to have the attributes 
mentioned in Section 1. Of these, descriptive RST already 
satisfies the first three to a significant degree: 
1. comprehensiveness: It has fit many different kinds 
of text, and has not failed to fit any kind of non-literary 
monologue we have tried to analyze. 
2. functionality: By means of the relation definitions, 
the theory says a great deal about what the text is 
doing for the writer (motivating, providing evidence, 
etc,). 
3. scale insensitivity: The recursiveness of schemas 
allows us to posit structural units at many scales 
between the clause and the whole text. Analysis of 
complete magazine articles indicates that the theory 
scales up well from the smaller texts on which it was 
originally developed. 
We See no immediate possibility of formalizing and 
programming the descriptive theory to create a programmed text 
analyzer. To do so would require reconciling it with mutually 
compatible formal theories of speech acts, lexical semantics, 
grammar, human inference, and social relationships, a collection 
which does not yet exist. Fortunately, however, this does not 
impede the development of a constructive version of RST for text 
generation. 
2.4. Developing a Constructive RST 
Why do we expect to be able to augment RST so that it is a 
formalizable and programmable theoretical framework for 
generating text? Text appears as it does because of intentional 
activity by the writer. It exists to serve the writer's purposes. Many 
11in another paper, we have shown that implicit communication arises from the 
use of the relations, that this communication is specific to each relation, and that 
as linguistic phenomena the relations and their implicit communication are not 
accounted for by particular existing discourse theories \[Mann & Thompson 83\]. 
373 
of the linguistic resources of natural languages are associated 
with particular kinds of purposes which they serve: questions for 
obtaining information, marked syntactic constructions for creating 
emphasis, and so forth. At the schema level as well, it is easy to 
associate particular schemas with the effects that they tend to 
produce: the Request schema for inducing actions, the Evidence 
schema for making claims credible, the/nform schema for causing 
the reader to know particular information, and so forth. Our 
knowledge of language in general and rhetorical structures in 
particular can be organized around the kinds of human goals that 
the linguistic resources tend to advance. 
The mechanisms of RST can thus be described within a 
more general theory of action, one which recognizes means and 
ends. Text generation can be treated as a variety of goal pursuit. 
Schemas are a kind of means, their effects are a kind of ends, and 
the restrictions created by the use of particular relations are a kind 
of precondition to using a particular means. 
Goal pursuit methods are well precedented in artificial 
intelligence, in both linguistic and nonlinguistic domains \[Appelt 
81, Allen 78, Cohen 78, Cohen & Perrault 77, Perrault & Cohen 
78, Cohen & Perrault 79, Newell & Simon 72\]. We expect to be 
able to create the constructive part of RST by mapping the 
existing part of RST onto AI goal pursuit methods. In particular 
computational domains, it is often easy to locate formal correlates 
for the notions of evidence, elaboration, condition, and so forth, 
that are expressed in rhetorical structure; the problem of 
formalization is not necessarily hard. 
At another level, we have some experience in using RST 
informally as a writer's guide. This paper and others have been 
written by first designing their rhetorical structure in response to 
stated goals. For this kind of construction, the theory seems to 
facilitate rather than impede creating the text. 
3. Comparing RST to Other Text 
Generation Research 
Given the mechanisms and example above, we can 
compare RST to other computational linguistic work on text 
generation. 12 The most relevant and well known efforts are by 
Appelt (the KAMP system \[Appelt 81\]), Davey (the PROTEUS 
system \[Davey 79\]), Mann and Moore (the KDS system \[Mann & 
Moore 80, Mann & Moore 81\]), McDonald (the MUMBLE system 
12Relating RST to the relevant /inguistic literature is partly done in \[Mann & 
Thompson 83\], and is outside the scope of this paper. However, we have been 
particularly influenced by Grimes \[Grimes 75\], Hobbs \[Hobbs 76\], and the work of 
McKeown discussed below. 
\[McDonald 80\]) and McKeown (the TEXT system \[McKeown 82\]). 
All of these are informative in other areas but, except for 
McKeown, they say very little about text organization. 
Appelt acknowledges the need for a discourse component, 
but his system operates only at the level of single utterances. 
Davey's excellent system uses a simple fixed narrative text 
organization for describing tic.tac.toe games: moves are 
described in the sequence in which they occurred, and 
opportunities not taken are described just before the actual move 
which occurred instead. Mann and Moore's KDS system 
organizes the text, but only at the whole-text and single-utterance 
levels. It has no recursion in text structure, and no notion of text 
structure components which themselves have text structure. 
McDonald took as his target what he called "immediate mode," 
attempting to simulate spontaneous unplanned speech. His 
system thus represents a speaker who continually works to 
identify something useful to say next, and having said it, recycles. 
It operates without following any particular theory of text structure 
and without trying to solve a text organization problem. 
McKeown's TEXT system is the only one of this collection 
that has any hint of a scale-insensitive view of text structure. It has 
four programmed "schemas" (limited to four mainly by the 
computational environment and task). Schemas are defined in 
terms of a sequence of text regions, each of which satisfies a 
particular "rhetorical predicate." The sequence notation 
specifies optionality, repeatability, and allowable alternations 
separately for each sequence element. Recursion is provided by 
associating schemas with particular predicates and allowing 
segments of text satisfying those predicates to be expressed using 
entire schemas. Since there are many more predicates than 
schemas, the system as a whole is only partially recursive. 
McKeown's approach differs from RST in several ways: 
McKeown's schemas are ordered, those of RST 
unordered. 
Repetition and optionality are specified locally; in RST 
they are specified by a general convention. 
McKeown's schemas do not have a notion of a 
nuclear element. 
McKeown has no direct correlate of the RST relation. 
Some schema elements are implicitly relational (e.g., 
an "attributive" element must express an attribute of 
something, but that thing is not located as a schema 
element). The difference is reduced by McKeown's 
direct incorporation of "focus." 
The presence of nuclear elements in RST and its diverse 
collection of schemas make it more informative about the 
functioning of the texts it describes. Its relations make the 
374 
connectivity of the text more explicit and contribute strongly to an 
account of implicit communication. 
Beyond these differences, McKeown's schemas give the 
impressio n of defining a more finely divided set of distinctions over 
a narrower range. The four schemas of TEXT seem to cover a 
range included within that of the RST Inform schema, which relies 
strongly on its five variants of the "elaboration" relation. Thus 
RST is more comprehensive, but possibly coarser.grained in 
providing varieties of description. 
Our role for text organization is also different from 
McKeown's. In the TEXT system, the text was organized by a 
schema-controlled search over thinas that are oermissible to sav. 
In constructive RST, text will be organized by goal pursuit, i.e., by 
ooal-based selection. For McKeown's task the difference might 
not have been important, but the theoretical differences are large. 
They project very different roles for the writer, and very different 
top-level general statements about the nature of text. 
Relative to all of these prior efforts, RST offers a more 
comprehensive basis for text organization. Its treatment of order, 
optionality, organization around a nucleus, and the relations 
between parts are all distinct from previous text generation work, 
and all appear to have advantages. 
4. Summary 
A text generation process must be designed around a 
theory of text organization. Most of the prior computational 
linguistic work offers very little content for such a theory. In this 
paper we have described a new theoretical approach to text 
organization, one which is more comprehensive than previous 
approaches. It identifies particular structures with particular ways 
in which the text writer is served. The existing descriptive version 
of the theory appears to be directly extendible for use in text 
construction. 
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