Clitics can be defined as prosodically defective function words. They can belong to a number of syntactic categories, such as articles, pronouns, prepositions, complementizers, negative adverbs, or auxiliaries. They do not generally belong to open classes, like verbs, nouns, or adjectives. Their prosodically defective character is most often manifested by the absence of stress, which in turn correlates with vowel reduction in those languages that have it independently; sometimes the clitic can be just a consonant or a consonant cluster, with no vowel. This same prosodically defective character forces them to attach either to the word that follows them (proclisis) or to the word that precedes them (enclisis); in some cases they even appear inside a word (mesoclisis or endoclisis). The word to which a clitic attaches is called the host. In some languages (like some dialects of Italian or Catalan) enclitics can surface as stressed, but the presence of stress can be argued to be the result of assignment of stress to the host-clitic complex, not to the clitic itself. One consequence of clitics being prosodically defective is that they cannot be the sole element of an utterance, for instance as an answer to some question; they need to always appear with a host.
A useful distinction is that between simple clitics and special clitics. Simple clitics often have a nonclitic variant and appear in the expected syntactic position for nonclitics of their syntactic category. Much more attention has been paid in the literature to special clitics. Special clitics appear in a designated position within the clause or within the noun phrase (or determiner phrase). In several languages certain clitics must appear in second position, within the clause, as in most South Slavic languages, or within the noun phrase, as in Kwakw'ala. The pronominal clitics of Romance languages or Greek must have the verb as a host and appear in a position different from the full noun phrase. A much debated question is whether the position of special clitics is the result of syntactic movement, or whether other factors, morphological or phonological, intervene as well or are the sole motivation for their position. Clitics can also cluster, with some languages allowing only sequences of two clitics, and other languages allowing longer sequences. Here one relevant question is what determines the order of the clitics, with the main avenues of analysis being approaches based on syntactic movement, approaches based on the types of morphosyntactic features each clitic has, and approaches based on templates. An additional issue concerning clitic clusters is the incompatibility between specific clitics when combined and the changes that this incompatibility can provoke in the form of one or more of the clitics. Combinations of identical or nearly identical clitics are often disallowed, and the constraint known as the Person-Case Constraint (PCC) disallows combinations of clitics with a first or second person accusative clitic (a direct object, DO, clitic) and a third person (and sometimes also first or second person) dative clitic (an indirect object, IO, clitic). In all these cases either one of the clitics surfaces with the form of another clitic or one of the clitics does not surface; sometimes there is no possible output. Here again both syntactic and morphological approaches have been proposed.