Blackwood 4NT asks for aces. Responder bids 5C with 0 or 4, 5D with 1, 5H with 2, and 5S with 3. A following 5NT asks for kings, but is never bid without partnership possession of 4 aces. Experts prefer cue bids over Blackwood if there is a possiblity of missing both the ace and king in a suit.
S: AKQxxx H: Ax D: AKJT C: x
S: JTx H: KQx D: Qxxx C: Qxx
W N E S 2C P 2D P 2S P 3S P 4N P 5C P 6S P/O