A version of Blackwood originally used in the Roman system but now rare. After 4NT, the responses are:
5♣ zero or three aces.
5♦ one or four aces.
The responses of 5♥, 5♠ and 5NT show two aces, either of the same colour, the same rank or the two other aces. The original school of thought was:
5♥ two aces of the same colour.
5♠ two aces neither of the same rank nor the same colour (ie spades and diamonds or hearts and clubs).
5NT two aces of the same rank.
Some players prefer the CRO principle, ie. 5♥ same Colour, 5♠ same Rank, 5NT the two Others. A player can then ask for kings in a similar way.