Book excerpt: While the comparative method is concerned with regularities in phonological change, grammaticalization theory deals with regularities of grammatical change. In an A-Z format, this 2002 book summarizes the most salient generalizations that have been made on the unidirectional change of grammatical forms and constructions. The product of ten years of research, World Lexicon of Grammaticalization provides the reader with the tools to show how different grammatical meanings can be related to one another in a principled way, how to deal with issues such as polysemy and heterosemy, or why certain linguistic forms have simultaneous lexical and grammatical functions. It covers several hundred grammaticalization processes, in each case offering definitions of lexical concepts, suitable examples from a variety of languages, and references to the relevant research literature. Indices organized by source and target concepts allow for flexible use, and the findings delineated in the book are relevant to students of language across theoretical boundaries.