Gerber

A 4♣ bid to ask partner how many aces he holds. The traditional responses are: 4♦, no ace; 4♥, one ace; 4♠, two aces; 4NT, three aces, and 5♣, four aces. 

4♦ can be used instead of 5♣ to show the rare holding of four aces. Experts strongly favor (62% in Bridge World Standard 2001) use of a modified responding scheme:

4♦ no ace or three aces.

4♥ one ace or four aces.

4♠ two aces. 

This is analogous to the responses to Roman Key Card Blackwood. 

As originally written (Rolling Gerber), the 4♣ bidder uses the next available bid to ask for kings on the same principle, but cannot use the agreed trump suit for this purpose. For example, 4♠ asks for kings over a response of 4♥, unless spades is the agreed trump suit, in which case 4NT becomes the king-asking bid. The modern tendency is to use 4♦ to show four aces along with 5♣ to ask for kings rather than the next higher bid. This helps remove ambiguity.

There may often be difficulty in distinguishing a conventional 4♣ bid from a natural one. Some players restrict the use of the convention to situations in which no suit has been genuinely bid (e.g., after a 1NT or 2NT opening, or a conventional 2♣ bid followed by 2NT or 3NT).

If 4♣ is to be used more generally, there are three possible rules a partnership can adopt:

(1) 4♣ is conventional unless it is a direct club raise.

(2) 4♣ is conventional unless clubs have been genuinely bid by the partnership.

(3) 4♣ is conventional if it is a jump bid, or if a suit has been specifically agreed. This is perhaps the best of these rules.

A partnership also has to consider how responder should act holding a void, or when there is interference bidding.

The convention named for John Gerber was invented in 1938 and is sometimes referred to as 4♣ Blackwood. The convention was devised earlier independently by Dr. William Konigsberger and Wim Nye, and published by them in Europe in 1936. Gerber can also be used after responder to a 1NT opening has used Stayman and found a major-suit fit (in which case 4♣ would ask for aces, 4♦ would be a balanced slam try with four trumps, 4NT would be quantitative with no fit). 

SOURCE: "Conventions", The Official ACBL Encyclopedia of Bridge, 7th ed. Horn Lake: American Contract Bridge League, Inc., 2011, page 288-289. Print