As indicated as follows, each CFG rule has the form: P L n …L 1 H R 1 …R m Here, P is the parent node, H is the head child of the rule, L n …L 1 and R 1 …R m are left and right modifiers of H respectively, and both n and m may be zero.
the parent node P depends on the head child H, this is what we call constituent dependency.