“Acol is the bridge bidding system that, according to The Official Encyclopedia of Bridge, is “standard in British tournament play and widely used in other parts of the world”. It is basically a natural system using four card majors and, most commonly, a weak no trump.” It is named after the Acol Bridge Club, previously located on Acol Road in London NW6, where the system started to evolve in the early 1930s. As a bidding system, Acol has the following characteristics: It is a natural system: most opening bids, responses and rebids are made with at least 4 cards in the suit bid, and most no trump bids are made with balanced hands. It is a four-card major system: only four-card suits are required to open 1♠ or 1♥ It is an approach forcing system: whether or not a bid is forcing (systemically requiring a response) depends on the previous bidding (“approach”). This is in contrast to level forcing systems, such as 2-over-1, where the level of the bid determines whether or not it is forcing. It makes extensive use of limit bids: limit bids describe the hand so closely, in terms of high card points (HCP) and shape, that the one who makes the limit bid is expected to pass on the next round, unless partner makes a forcing bid. Understanding and correct use of limit bids and forcing bids is fundamental to applying the system: all no trump bids below the level of 4NT are limit bids, as are all suit bids that merely repeat a suit already bid by the partnership; changes of suit may be forcing or not depending on the approach bids. The level of the 1 NT opening bid influences other bids: the normal choice is between a “weak no trump” (12-14 HCP) and a “strong no trump” (15-17 HCP). If using a weak no-trump, this is the only “fully natural” bidding system which does not require a “short club” or “prepared” club/diamond bid with less than 4 cards. All 1 of a suit opening bids then promise at least 4 cards in the bid suit.”from http://en.wikipedia.org/wiki/Acol