This book is the first major cross-linguistic study of 'flexible words', i.e. words that cannot be classified in terms of the traditional lexical categories Verb, Noun, Adjective or Adverb. Flexible words can - without special morphosyntactic marking - serve in functions for which other languages must employ members of two or more of the four traditional, 'specialised' word classes. Thus, flexible words are underspecified for communicative functions like 'predicating' (verbal function), 'referring' (nominal function) or 'modifying' (a function typically associated with adjectives and e.g. manner adverbs). Even though linguists have been aware of flexible world classes for more than a century, the phenomenon has not played a role in the development of linguistic typology or modern grammatical theory. The current volume aims to address this gap by offering detailed studies on flexible word classes, investigating their properties and what it means for the grammar of a language to have such a word class. It includes new cross-linguistic studies of word class systems as well as original descriptive and theoretical contributions from authors with an expert knowledge of languages that have played - or should play - a role in the debate about flexible word classes, including Kharia, Riau Indonesian, Santali, Sri Lanka Malay, Lushootseed, Gooniyandi, and Late Archaic Chinese.

This handbook explores multiple facets of the study of word classes, also known as parts of speech or lexical categories. These categories are of fundamental importance to linguistic theory and description, both formal and functional, and for both language-internal analyses and cross-linguistic comparison. The volume consists of five parts that investigate word classes from different angles. Chapters in the first part address a range of fundamental issues including diversity and unity in word classes around the world, categorization at different levels of structure, the distinction between lexical and functional words, and hybrid categories. Part II examines the treatment of word classes across a wide range of contemporary linguistic theories, such as Cognitive Grammar, Minimalist Syntax, and Lexical Functional Grammar, while the focus of Part III is on individual word classes, from major categories such as verb and noun to minor ones such as adpositions and ideophones. Part IV provides a number of cross-linguistic case studies, exploring word classes in families including Afroasiatic, Sinitic, Mayan, Austronesian, and in sign languages. Chapters in the final part of the book discuss word classes from the perspective of various sub-disciplines of linguistics, ranging from first and second language acquisition to computational and corpus linguistics. Together, the contributions showcase the importance of word classes for the whole discipline of linguistics, while also highlighting the many ongoing debates in the areas and outlining fruitful avenues for future research.