3.1.1 Rewrite rules and some terminology

The meaning of the ‘bar’ can be seen in terms of the notion of projection. We can imagine a phrase as a three-floored building, with a ground floor, a first floor and a top floor. On the ground floor we have the head, which is not built on top of anything – it is an unprojected element. Often heads are called zero level projections, to indicate that they are not projected from anything. This can be represented as X⁰.