Not everyone agreed with Chomsky. During the 1960s and 1970s there was a so-called “Linguistics War” between those who thought syntax was most important and those who believed in the importance of semantics. The Chomskyan syntax group won and developed a theory in which syntax is relatively independent from semantics. Grammar, therefore, was to be central to understanding how human language ability works (at least for Chomsky’s followers). By understanding grammar, it was claimed, we could understand the mysterious workings of the human brain.
With an Operator node that could be used to decide the type of sentence, it naturally made people think about more movement possibilities. So, for example, in the simple sentence below, we have invisible movement to the aux node.
Now, maybe we can move some kind of tense element further in order to make a question sentence.
These mysterious movement operations became central to Chomsky’s ideas about language. While the idea that things are moving around in the grammar seemed a little strange, this could be explained as part of our unique, universal grammatical ability.
The next important change concerned the categories employed in syntactic analysis. This movement was to become known as X-bar theory. This is sometimes written as X’ or with a line over the X. This just meant that all categories had (basically the same) levels of structure. X” was the highest level, X’ was the next, and X was the lowest level.
Remember our aux node? We might imagine that the inflectional properties of a verb (tense, aspect, etc.) move to this node for some reason. Now this category had to follow X-bar theory and project the same structure as the other categories. This became known as the Inflectional Phrase.