Responding to a 1 level opening

• Bear in mind that partner may be on the light side, but explore for major fit as you would in any system. You should pass on 0-5, and an occasional bad six. Having made a call and heard a simple 2 level rebid you are more likely to pass in Acol than in N American methods
• Try to express a major at the one level if you can. Remember that opener calls 1 when 4-4 majors. Occasionally with 5 spades, 4 hearts and few values you may have to lie (treat as 4-4) unless strong enough (10+) to bid twice.
• Direct 1NT denies a bypassed major, and is 8-10 opposite 1, otherwise 6-9. As in other countries: responder's new suit after bidding 1NT is weak and to play!
• Passing on up to 6 HCP is safer at Acol, as opener has not called a strong two
• 2 over 1 is called quite light 9+ or even 8 with a fine suit. 2 always shows five cards Minors can be 4 or even 3 if you can handle the consequences. "Modern" parnerships commonly agree 2/1=10+.
• 1 any - 2NT is natural (no major again) with good 10 to 12. Try to avoid this call, as you can develop and make it next round
• 1 any - 3NT is 13-15 balanced. Again better avoided in favour of approach forcing. "Stultifying" - (Terence Reese).
• 1M - 2M shows about 5-9. Ideally 4 card support, but often called on 3 trumps and a ruffing value. "Support with support". Opener may bid 2NT or 3NT with a good hand and only 4 trumps, and responder should then exercise judgement.
  Trial bids (say 1 - 2 - 3*) show length 3+ in the strain with at least 2 losers there. Partner is asked for help. He should call game with 2 top honours in the suit or (better) good trumps and an ability to ruff. With a maximum 2 level raise game may still be on, but beware quick losers in the trial suit. Controls elsewhere should then be a consideration. Either partner calling NT in this situation suggests we might have only 7 trumps
• 1m - 3m is 10-12, always 4 card support. New suit is now a stop - probing 3NT (or advance cue if 3NT later removed)
• 1M - 3M is 10-12, always 4 card support. New suit now is a cue bid (ace or void by default) suggesting slam
• 1M - 4/ are variously played as splinter, cue, 1960's Swiss (sound raise to 4, two aces) or the old-fashioned default of "to play". Opposite pickup I'd assume a splinter, but ideally you should agree this. Splinters in majors and mid auction aren't for pick-up!
• Strong hands An immediate jump shift (strong jump shift) is traditionally 16+, and you may meet the occasional dinosoar who does this in a shortish suit just to show points. The great majority of Acol players can nowadays be expected to 14+ points and visions of 5 level+, and either an excellent single suit, a fair-good 5 cards and good support for partner.
  Other hands should just bid out their shape naturally. A 3 level new suit, or 4th suit at any stage is forcing. A "responder's reverse" is GF and should always be 5-4. A common route with say 5431 and 3 cards for partner is to bid out your shape, game on the last round. Playing 4 card majors - that clever splinter may not work out too well if the fit is 4-3!
  Be particularly cautious when either partner has made a limit bid. This includes most 1-3NT rebids and simple or 3 level rebids of one's own suit. Some players will now pass returns to previosly bid suits. However in the modern style any move at 3+ level over a limit bid (commonly 2NT) should lead to game, and few good players would pass 3 of a major here. If in doubt use jump, new suit or 4sF for clarity.