A theory of syntactic structure, proposed by Chomsky (1995), according to which syntactic constituents have no category labels, and different levels of projection are not distinguished by means of notation.
Radford (2004:94–95) proposes a version of bare phrase structure which does not distinguish levels of projection, but does include category labels. Thus all phrase-structure rules are of the form
X → X Y or X → Y X.
Radford’s notation is used in this online dictionary.