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 as follows.
|5 || 0 aces|
|5 ||Shows the ace of diamonds, and no other aces|
|5 ||Shows the ace of heart, and no other aces|
|5 ||Shows the ace of spades, and no other aces|
|5NT || Any two aces|
|6 || Shows the ace of clubs, and no other aces|
Opener's rebids are natural. His hand must be good enough, of course, to play at the five-level opposite an aceless, trickless hand.