The three purposes of bidding are to describe your hand, find the fit AND agree the contract.
An opening hand will normally have at least 12 points.
In ACOL we can open with a four card major but it is preferable to open a five card suit.
Many hands are unbalanced and have one suit longer than the other. Where there is a 5 card and a 4 card suit, bid the longest first. On your second bid you will be able to show your second suit and your partner will know which is the longer suit.
If you have two five card suits bid the senior one first.
If you have 4 spades and 4 hearts bid hearts first to allow your partner the chance to bid spades at the lowest level.
Where you have a hand with a lot of cards in one or two suits but less than 12 points you can use the Rule of Twenty. Add the points in your hand to the number of cards in your two longest suits, if the total is twenty or more you may open your longest suit.
STRONG UNBALANCED HANDS - 16+ HCP
See Opener‘s Rebid with a Strong Hand.
With balanced (flat) hands with 12 to 14 points bid 1NT. This is a precise description of your hand so do not deviate from the rule. 11 or 15 points do not merit 1NT.
STRONG BALANCED HAND
With a balanced hand and 15 to 19 HCP bid a four card suit (H before S before minors). If you bid again you can bid NT. This show a strong balanced hand. With 15-16 rebid NT at the lowest level.
with 17-18 HCP rebid with a jump
with 19 HCP rebid 3NT
With a balanced hand and 20 to 22 HCP bid 2NT.
For hands with 23 points or more bid 2C. The response to this bid is always at least 2D so that the opener can bid again. An opening of 2C is game forcing. This means bidding must continue until a game contract is found. The only exception is if the opener rebids 2NT which shows a flat hand. In this case if the responder has 2 points or less the bid can be passed.
Response to 2C : 2D (relay 0-7 HCP), 2H, 2S, 2NT, 3C,3D, WITH 8+, 3NT with 11-13 HCP balanced.
PREEMPTIVE OPENING BIDS
In Modern Acol most people now play 3 Weak 2s.
Where you have a weak hand (6 to 9 points) and and you are not vulnerable and you DON’T have another four card Major - you can open at the 2, 3 or 4 level. This is called a Pre-emptive Bid and has the effect of making it difficult for your opponents to find their contract.
With a 6 card suit you can open at the 2 level. 2D or 2H OR 2S.
REMEMBER 2C is strong - see above.
With a 7 card suit you can open at the 3 level. 3C or 3D or 3H or 3S.
With an 8 card suit you can open at the 4 level. 4H or 4S
If you make a preemptive opening, do not bid again - you have described your hand.