Overview
Complex phrases could theoretically be represented as so-called 'flat structures'. Here is an example:
However, in applying various constituent tests, we can show that a hierarchical structure is more adequate:
The theory that accounts for such representations is referred to as X'-Theory or X'-Syntax.
X'-Syntax (pronounced X-bar) was introduced in the mid 1970s and has become an integral part of generative grammar. The central idea of this theory of phrase structure is that all phrasal and clausal categories are organized around a head according to a common (hierarchical) scheme involving the lexical level (X), one or more intermediate (bar-) levels (X'), and the phrasal level (XP) at the top.
The following constituents will be discussed in detail:
CP - IP - NP - DP - VP - PP - AP
The basic terminology will also be discussed briefly.