Generation of empty categories
Not all empty categories enter the derivation of a sentence at the same point. Both DP-trace and WH-trace, as well as all of the null heads, are only generated as the result of movement operations. Trace refers to the syntactic position which is left after something has moved, helping to explain how DP-trace and WH-trace get their names. What is meant by "trace" is that there is a position in the sentence that holds syntactic content in the deep structure, but that has undergone movement so that it is not present at the surface structure. Conversely, both "PRO" and "pro" are not the result of movement and must be generated in the deep structure.  In both the government and binding and minimalism frameworks, the only method of base-generation is lexical insertion. This means that both "PRO" and "pro" are held to be entries in the mental lexicon, whereas DP-trace and Wh-trace, and the null heads are not categories in the lexicon.