*If you live in the UK, Ireland, Australia or New Zealand Acol is the most widespread 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♥, unlike Standard American and many other systems where five-card suits are typically required.
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 1NT 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).
All 1 of a suit opening bids then promise at least 4 cards in the bid suit
Notrump follow-up conventions include Stayman, Jacoby transfers Blackwood and Gerber Convention.