A-over-A

In generative grammar, the A-over-A principle says that if a rule ambiguously refers to A in a structure of the form of (i), the rule must apply to the higher, more inclusive, node A (Chomsky 1964).

Examples
This principle prevents extraction of the NP Africa out of the NP my trip to Africa in which it is included in (ii)b, but allows the more inclusive NP to be fronted in (ii)c.

(i)	 ... [A ... [A ...  (ii) a I won't forget [NP my trip to [NP Africa ]]. b *Africa, I won't forget my trip to c my trip to Africa, I won't forget

Comment
More recently, the A-over-A principle has been reduced to principles such as the ECP.

Link
Utrecht Lexicon of Linguistics