Commentary on Kaplan and Kay 
Graeme Ritchie I 
(University of Edinburgh) 
To appreciate this article fully, it is essential to understand the historical context into 
which it fits, and which it has to some extent created. Although formally published 
for the first time here, it is already an extremely influential and classic piece of work. 
Finite-state machines, in one form or another, have been used for the description 
of natural language since the early 1950s, with the extension to transducers appearing 
in the 1960s. After Chomsky's stern condemnation of the adequacy of finite-state ma- 
chines for describing sentence structures, they virtually disappeared from mainstream 
theoretical linguistics. Within computer science, they continued to be a standard for- 
malism, although transducers were not accorded the same detailed algebraic attention 
as simple automata. 
Phonologists, meanwhile, were inventing a variety of rule mechanisms that were 
(with rare exceptions) only partly formalized. Superficially, most of these systems (as 
typified by those of Chomsky and Halle) appeared to have little to do with finite-state 
machines. Indeed, their notations tended to suggest that the rules had much more 
than finite-state power. 
Kaplan and Kay have integrated these two streams of work--algebraic treatment 
of automata in computer science, and phonologically-motivated formalisms within 
linguistics--and their results should feed back productively into both subfields. The 
framework they have established allows the comparison of different competing for- 
malisms in a rigorous manner, and permits the exploration of the formal limitations or 
capabilities of rule notations that were previously more like expository devices than 
formally defined systems. 
What may not be clear to the casual reader is that this work has been developed 
over many years, and early versions of it have already escaped into the computational 
linguistics community in less prominent forums. In this way it has already affected 
the course of research into phonological/morphological formalisms. Perhaps the most 
notable (and in its turn, influential) development has been Koskenniemi's two-level 
morphology, which has been successfully applied to the morphology of a very wide 
number of languages. Koskenniemi's ideas are a direct development of Kaplan and 
Kay's, as explained in Section 7 of the paper here. 
The theory of regular relations and finite-state transducers should not be viewed 
as a mere re-formalisation of 1960s linguistics. As well as its relevance to the two-level 
model, Kaplan and Kay suggest that it may also throw light on the formal properties 
of autosegmental phonology. Although the ideas were first circulated about 15 years 
ago, they are still of central relevance to computational phonology today. 
References 
Chomsky, N. (1957). Syntactic Structures. 
Mouton. 
Koskenniemi, K. (1984). "A general 
computational model for word-form 
recognition and production." In 
Proceedings, l Oth International Conference on 
Computational Linguistics/22nd Annual 
Meeting of the ACL. 178-181. Stanford, CA. 
Chomsky, N., and Halle, M. (1968). The 
Sound Pattern of English. Harper and Row. 
1 Department of Artificial Intelligence, University of Edinburgh, Edinburgh EH1 1HN, Scotland. 
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