Gerber

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

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

After a bid of NT, 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.