The term “part of speech” is a traditional one that has been in use since grammars of Classical Greek (e.g., Dionysius Thrax) and Latin were compiled; for all practical purposes, it is synonymous with the term “word class.” The term refers to a system of word classes, whereby class membership depends on similar syntactic distribution and morphological similarity (as well as, in a limited fashion, on similarity in meaning—a point to which we shall return). By “morphological similarity,” reference is made to functional morphemes that are part of words belonging to the same word class. Some examples for both criteria follow: The fact that in English, nouns can be preceded by a determiner such as an article (e.g.,
the apple) illustrates syntactic distribution. Morphological similarity among members of a given word class can be illustrated by the many adverbs in English that are derived by attaching the suffix –ly, that is, a functional morpheme, to an adjective (quick, quick-ly). A morphological test for nouns in English and many other languages is whether they can bear plural morphemes. Verbs can bear morphology for tense, aspect, and mood, as well as voice morphemes such as passive, causative, or reflexive, that is, morphemes that alter the argument structure of the verbal root. Adjectives typically co-occur with either bound or free morphemes that function as comparative and superlative markers. Syntactically, they modify nouns, while adverbs modify word classes that are not nouns—for example, verbs and adjectives.
Most traditional and descriptive approaches to parts of speech draw a distinction between major and minor word classes. The four parts of speech just mentioned—nouns, verbs, adjectives, and adverbs—constitute the major word classes, while a number of others, for example, adpositions, pronouns, conjunctions, determiners, and interjections, make up the minor word classes. Under some approaches, pronouns are included in the class of nouns, as a subclass.
While the minor classes are probably not universal, (most of) the major classes are. It is largely assumed that nouns, verbs, and probably also adjectives are universal parts of speech. Adverbs might not constitute a universal word class.
There are technical terms that are equivalents to the terms of major versus minor word class, such as content versus function words, lexical versus functional categories, and open versus closed classes, respectively. However, these correspondences might not always be one-to-one.
More recent approaches to word classes don’t recognize adverbs as belonging to the major classes; instead, adpositions are candidates for this status under some of these accounts, for example, as in Jackendoff (1977). Under some other theoretical accounts, such as Chomsky (1981) and Baker (2003), only the three word classes noun, verb, and adjective are major or lexical categories. All of the accounts just mentioned are based on binary distinctive features; however, the features used differ from each other. While Chomsky uses the two category features [N] and [V], Jackendoff uses the features [Subj] and [Obj], among others, focusing on the ability of nouns, verbs, adjectives, and adpositions to take (directly, without the help of other elements) subjects (thus characterizing verbs and nouns) or objects (thus characterizing verbs and adpositions). Baker (2003), too, uses the property of taking subjects, but attributes it only to verbs. In his approach, the distinctive feature of bearing a referential index characterizes nouns, and only those. Adjectives are characterized by the absence of both of these distinctive features.
Another important issue addressed by theoretical studies on lexical categories is whether those categories are formed pre-syntactically, in a morphological component of the lexicon, or whether they are constructed in the syntax or post-syntactically. Jackendoff (1977) is an example of a lexicalist approach to lexical categories, while Marantz (1997), and Borer (2003, 2005a, 2005b, 2013) represent an account where the roots of words are category-neutral, and where their membership to a particular lexical category is determined by their local syntactic context. Baker (2003) offers an account that combines properties of both approaches: words are built in the syntax and not pre-syntactically; however, roots do have category features that are inherent to them.
There are empirical phenomena, such as phrasal affixation, phrasal compounding, and suspended affixation, that strongly suggest that a post-syntactic morphological component should be allowed, whereby “syntax feeds morphology.”