Bidding and conventions
Acol
Acol is the bidding system most British players use, built on four-card majors and a 12–14 notrump. Hold ♠ A Q 7 4 ♥ K 8 3 ♦ A J 6 ♣ 9 5 2 and an Acol player opens 1♠ where an American opens 1♣.
Four-card majors
An Acol player opens a four-card major in preference to a three-card minor. With 4-4 in the majors and a minimum, open the lower one, 1♥, which leaves room to find spades later.
14 points. Acol opens the major. Standard American opens 1♣ and hopes to bid spades later.
13 points, 4-4 in the majors. Open the lower one so partner can show spades at the one level.
What it buys: 4-4 major fits are found at once. In a five-card major system the same fit can take several bids to find, or get missed.
What it costs: the opening bid no longer guarantees five, so responder needs four-card support to raise, and the partnership occasionally plays a 4-3 fit.
The weak notrump
1NT shows 12–14 balanced. That makes it the most frequent opening bid in the system: a great many ordinary hands are balanced 12-counts, and in Acol they all get described in one call.
This changes the rebids. Because the weak notrump takes the 12–14 balanced hands, a one-of-a-suit opening followed by a 1NT rebid shows 15–16. In Standard American the same rebid shows 12–14.
The other notrump rebids move up with it, so the weak notrump and its rebid structure have to be played together.
The two-level openings
Classic Acol used strong Acol two-bids: 2♦, 2♥ and 2♠ all showed powerful single-suited hands about a trick short of game, forcing for one round.
Modern Acol has mostly abandoned them in favour of weak twos, with 2♣ as the single strong artificial opening.
If you learnt from an older Acol book, check with a new partner which kind of two-bids you play.
Acol or Standard American?
Mostly a question of where you play. Acol is standard in Britain, Ireland, Australia and New Zealand; Standard American and two over one dominate in North America. Online you'll meet both, and many robots bid a five-card major system by default.
Neither is clearly better. Four-card majors find more 4-4 fits; five-card majors give responder more reliable information. The weak notrump describes more hands in one bid and is riskier when doubled.
Whichever you choose, agree it with partner. If one of you opens four-card majors and the other expects five, you'll keep misjudging your trump fits.
Where the name comes from
From the Acol Bridge Club in Acol Road, Hampstead, where the system was developed in the 1930s by a group including Maurice Harrison-Gray, Jack Marx and S. J. Simon.
It was meant as a natural, judgement-led alternative to the heavily systemised American methods of the period, with fewer rules and more room for judgement. Its supporters still like it for that.