Gerber

The Gerber convention, like Blackwood, is a way of determining the number of aces when looking for a slam.  

You need 33 HCPs to make 6NT.

You need 35 HCPs to make 7NT.

Gerber is only used when the partnership has agreed that the contract will be in no trumps.

After an opening bid of 1NT or 2NT a response of 4C asks for aces. 

The response to 4C is:

4D = nil or 4 aces

4H = 1 ace

4S = 2 aces

4NT = 3 aces

After this round of bidding a bid of 5C may be used to ask for kings.

The response to 5C is:

5D = nil or 4 kings

5H = 1 king

5S = 2 kings

5NT = 3 kings

The advantage of using gerber with a NT contract, is that it is easy to stop the bidding short of a slam, if the number of aces is not sufficient.