An ACOL 4NT opening bid shows an extremely strong hand, usually a one-suiter, that only needs to hear about which aces (if any) partner holds in able to bid a slam. The responses to the ACOL 4NT are:

5	0 aces
5	Ace of diamonds only
5	Ace of hearts only
5	Ace of spades only
5NT	Any two aces
6	Ace of clubs only

Opener's rebids are natural. His hand must be good enough, of course, to play at the five-level opposite an aceless, trickless hand.