Culbertson 4-5 Notrump

Used both to show and ask for aces. A bid of 4NT shows three aces or two aces and the king of a fitted suit. The responses are:

5 of the lowest bid suit shows no aces
5 of any other suit shows that ace

5NT shows 2 aces, or one ace and all the kings of suits with found fits.

6 of the lowest suit in response shows that ace.