 In spite of its name, the ECP applies to only two of the four types of null DPs.Specifically, it applies to DP- and Wh-traces, but not PRO and "pro".The chief function of the ECP is to place constraints on the movement of categories by the rule of move α; it effectively allows a tree structure to "remember" what has happened at earlier stages of a derivation, and it can be seen as GB's version of the older derivational constraints.