X-Bar Theory and Constituent Structure

General claim of X-Bar theory:
The words in sentences (and phrases) have not just a linear order but also a hierarchical structure.

I. Hierarchical structures

In generative syntax, hierarchical structures are organised in such a way that two syntactic objects are joined to form a larger syntactic object (unit, constituent).
Hierarchical structures are represented in tree diagrams and syntactic objects or units are then referred to as nodes (cf. A, B, C below).

Larger syntactic units (such as C above) can join with another syntactic object or another complex syntactic unit to form a new unit. Note that not more than two syntactic objects can be joined to form a new object, resulting in binary branching structures.

II. Structural relations

The hierarchies in tree diagrams can be expressed with reference to structural relations. The basic structural relations are defined below:

(a) Motherhood:
A node A is the mother of node B, if it directly contains it. B is then referred to as A's daughter.
Example: In the tree diagram above C is the mother of A and B, and G is the mother of E and F.

(b) Sisters/Sisterhood:
A node A is node B's sister if it has the same mother as B.
Example: In the tree diagram above A and B are sisters.

(c) Dominance:
A node A dominates node B if it is higher up in the tree than A and contains it.
Example: In the tree diagram above G dominates all other nodes since it contains them.

Principles of X-Bar Theory
The X-Bar Model: Lexical Categories
X-Bar Theory and Constituent Tests

Exercises on Constituent Structure