A version of Blackwood in which the king of the agreed trump suit counts as a fifth ace, also known as Keycard Blackwood. In response to 4NT:
5♣ shows zero or four aces.
5♦ shows one or five aces.
5♥ shows two aces.
5♠ shows three aces.
Except that 5♦ might show five aces, the responses are the same as those to normal Blackwood.