One of these borderland creatures is something that linguists call a clitic. Clitics are small grammatical elements that cannot occur independently and therefore cannot really be called free morphemes. But they are not exactly like affixes either. In terms of their phonology, they do not bear stress, and they form a single phonological word with a neighboring word, which we will call the host of the clitic. However, they are not as closely bound to their host as inflectional affixes are; frequently they are not very selective about the category of their hosts. Those clitics that come before their hosts are called proclitics, those that come after their hosts enclitics.

Two types of clitics are often distinguished: simple clitics and special clitics. Anderson (2005: 10) defines simple clitics as “unaccented variants of free morphemes, which may be phonologically reduced and subordinated to a neighboring word. In terms of their syntax, though, they appear in the same position as one that can be occupied by the corresponding free word.” In English, forms like -ll or -d, as in the sentences in (13), are simple clitics:

In these sentences, -ll and -d are contracted forms of the auxiliaries will and would, and they occur just where the independent words would occur – following the subject I and before the main verb. Like affixes, they are pronounced as part of the preceding word. Unlike affixes, they do not select a specific category of base and change its category or add grammatical information to it. Contracted forms like -ll or -d in English will attach to any sort of word that precedes them, regardless of category:

In (14a) -ll is cliticized to the adverb there, and in (14b) -d is cliticized to the verb know.

Special clitics are phonologically dependent on a host, as simple clitics are, but they are not reduced forms of independent words. Compare the example in (15) from French:

Although the object pronoun le in French is written as a separate word, it is phonologically dependent on the verb to its right; in other words, the object pronoun and the verb are pronounced together as a single phonological word. There is no independent word that means ‘him’ in French. So le and the other object pronoun forms in French are special clitics.

Clitics are of interest both to syntacticians and to morphologists precisely because they have characteristics both of bound morphemes and of syntactic units. Like bound morphemes, they cannot stand on their own. But unlike morphemes, they are typically unselective of their hosts and have their own independent functions in syntactic phrases.