The X'-Scheme

The X'-scheme defines three types of categories:

  • Phrasal categories (e.g. CP, NP, VP etc.)
  • Intermediate categories (e.g. C', N', V')
  • Lexical categories (e.g. Comp, N, V)

All phrasal categories follow a common scheme. They consist of an obligatory head (N' in NP, V in V' etc.) and an optional specifier (PP in V', AP in N' etc.). In other words, X'-syntax exhibits strict binary branching. Each category has a maximum of two daughters.

This scheme is language-independent; only the linear ordering of head and specifier varies across languages:

  • head-first languages (e.g. Spanish):


  • head-final languages (e.g. Japanese)


  • mixed-type languages

The SPEC-node is generally optional. By convention, it can be dropped from the tree diagram if it is not filled.