Meaning of the X-bar item in X-bar theory?

Question

In X-Bar theory, what meaning do the X' nodes themselves represent?

I (believe I) understand the meaning of the XP items: they are the constituent syntactic category phrases. The constituents of a sentence that have syntactic function are XPs (is that correct?)

I also believe I understand the X items themselves, the heads of the XPs. It is the lexical item that gives the constituent it is in its syntactic category, based on its lexical category.

However, how can I understand what the X' nodes/items/subtrees are meant to represent?

Answer 1

The best you can say is, X' items are an intermediate level. That's really the thing that defines them. You can say they have the feature "bar=1", if you want, which is kind of tautological: "X' items are X' items".

You can say they represent some constituents being attached more closely to the head than other constituents. That might be a more intuitive way to phrase "they're an intermediate level".

In some variants, where X' levels aren't postulated unless actually needed (that is, there's no N' level in an NP consisting of a single noun), you can say that an X' element is not minimal (the head has c-selected something) and not maximal (it still needs to c-select something more). That's a little bit better. But classical X-bar theory doesn't do this; a single noun still has an N' level, even if it has no selectional features.