Twenty Years of Two-Level Morphology

ESSLLI 2001 Special Event organised by Lauri Karttunen, Kimmo Koskenniemi and Gertjan van Noord

The panelists



Twenty years ago morphological analysis of natural language was a challenge to computational linguists. Simple cut-and-paste programs could be and were written to analyze strings in particular languages, but there was no general language-independent method available. Furthermore, cut-and-paste programs for analysis were not reversible, they could not be used to generate words. Generative phonologist of that time described morphological alternations by means of ordered rewrite rules, but it was not understood how such rules could be used for analysis.

This was the situation in the spring of 1980 when Kimmo Koskenniemi came to a conference on parsing that Lauri Karttunen had organized at the University of Texas at Austin. Also at the same conference were two Xerox researchers from Palo Alto, Ronald M. Kaplan and Martin Kay. The four K's discovered that all of them were interested and had been working on the problem of morphological analysis. Koskenniemi went on to Palo Alto to visit Kay and Kaplan at PARC.

This was the beginning of Two-Level Morphology, the first general model in the history of computational linguistics for the analysis and generation of morphologically complex languages. The language-specific components, the lexicon and the rules, were combined with a runtime engine applicable to all languages.

The key insight was that lexicons and rules for morphological alternations represent regular relations. Consequently, they can be compiled into finite-state transducers. And these transducers can be combined with one another to yield, in the limit, a single transducer that maps all valid lemmas of the language into all of their proper realizations, and vice versa.

The finite-state paradigm has been very popular and hugely successful in natural language processing. This panel will bring together a number of senior researchers in this field for a retrospective look at the past and to propose future directions.