Acol is a bridge bidding system. It is the name of a road in Hampstead, London, where there was a bridge club in which the system started to evolve in the 1930s. It was popularised in Britain by Iain Macleod in his book "Bridge is an Easy Game", published in 1952. The Acol system is continually evolving but the underlying principle is to keep the bidding as natural as possible. It is common in the British Commonwealth but rarely played in America.
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Bidding system is set of agreements and conventions in bidding between two partners in contract bridge. A bidding system defines the meaning of every possible bid by each partner, and presents a codified language which allows the partners to find out about each other's holdings and determine the correct contract.