What is "gerber convention"

Gerber is a contract bridge convention devised by Dr. William Konigsberger and Win Nye from Switzerland who published it in 1936; John Gerber of Texas introduced it to North America in 1938 where it was named after him. It is similar to Blackwood but uses 4 instead of 4NT as an "asking bid" to inquire about the number of aces held by partner; a further asking bid may follow to inquire about the number of kings held.

Gerber is used primarily after notrump openings, responses, and rebids, making it a complement to Blackwood rather than a replacement. Some club players also use it after suit bidding, but most experts do not recommend this.