In X-bar theory and other grammatical theories that incorporate it, an inflectional phrase or inflection phrase (IP or InflP) is a functional phrase that has inflectional properties (such as tense and agreement). An inflectional phrase is essentially the same as a sentence, but reflects an analysis whereby a sentence can be treated as having a head, complement and specifier, like other kinds of phrases. Inflectional morphology, therefore, occurs after sentence construction is complete.
An inflectional phrase is a phrase that contains as its head an abstract category called Infl (short for 'inflection'). The Infl head bears inflectional properties such as tense and person, and may or may not be realised as separate words in the surface representation of the phrase. The other usual components of the IP are a verb phrase (VP), which is the complement of the phrase, and a noun phrase (NP), which is structurally the specifier of the phrase, and serves as the subject of the phrase. In this analysis, every simple sentence (i.e. one that is not coordinated) is an IP.
In some analyses, separate levels of phrase structure are proposed for different kinds of inflection. For example, there may be posited an agreement phrase (AgrP), with an Agr head that bears inflectional properties for verb agreement with the subject. There can also be direct object and indirect object agreement phrases (AgrOP, AgrDOP, AgrIOP), for languages in which verbs may exhibit agreement with an object. Other types of inflection may be encapsulated in a tense phrase (TP) for grammatical tense, aspect phrase (AspP) for grammatical aspect, and so on.
The postulation of such a multiplicity of categories has been criticized on the grounds that they appear not to be universal, many being found in only a minority of languages.This criticism is furthered by research that suggests that inflectional phrase can represent the differentiating mental and semantic processing layers in mind regarding language. These studies show that language learners have been found to take different avenues or strategies in IP structure-building.