Chapter 8 introduced a theory that the syntax component of our mental grammar is quite simple: It consists of only two operations, MERGE and MOVE. The operation MERGE does exactly one thing, namely, it combines words and features to create x-bar structures. X-bar theory claims that every phrase in every sentence in every language of the world is an x-bar phrase. There are only two kinds of things in x-bar phrases, heads and other phrases. Inside a given phrase, another phrase can occupy the position of complement, specifier, or adjunct. The structure that MERGE generates is the Deep Structure of a sentence, which exists in our mind. The operation MOVE takes the Deep Structure and moves some words or phrases to a new position in the Surface Structure of the sentence, which is the form of the sentence that we speak and observe. When MOVE operates, it always moves heads to other head positions, and phrases to other phrase positions. When wh-phrases move, their Surface Structure position is always SpecCP. And the very useful notation of tree diagrams allows us to illustrate how Deep Structures and Surface Structures are represented in our minds.