One of the oldest bridge Conventions is Blackwood.

When you have determined that the partnership has enough combined strength to be in a slam contract you may still be worried about the possibility that the opponents have 2 Aces out against you. The Blackwood Convention allows you to ask partner how many Aces he holds.

A 4NT bid by you asks partner to respond according to the following:
  • 5♣ : 0 or 4 Aces (if you can't tell which you shouldn't be asking)
  • 5 : 1 Ace
  • 5 : 2 Aces
  • 5♠ : 3 Aces
If you find that your side holds all 4 of the Aces, you can then bid 5NT, asking for partner's Kings.
The responses will be similar, but at the 6-level.
  • 6♣ : 0 or 4 Kings
  • 6 : 1 King
  • 6 : 2 Kings
  • 6♠ : 3 Kings
A similar convention, Gerber, uses a 4♣ bid to start the sequence. The Gerber summary is a little further down the menu list.

A more complicated version of Blackwood, RKC Blackwood, is even further down the menu.

Combined Blackwood / Gerber lesson available.