There are 2 starting points to any possible contract: 1NT or a bid of a suit. With 1NT partner is limited to exactly 12 -14 HCP and you base your actions on that inherent knowledge. You can pass 1NT with as many as 10 HCP whereas after a 1 level suit opening that would be criminal. This is because when partner opens 1 of a suit they may have from 10 to 20 HCP and we just don’t yet know, so we struggle to keep the bidding open for at least one round. (I'll leave discussions of how to respond after weak 2 or 3 bids for another time.)
Responding with 6 - 9 HCP
The weak range:
1) Always, always, always, ALWAYS respond with 6 HCP (assuming no overcall). Always, always, always, ALWAYS show a 4 card major at the 1 level if you can.
Do NOT, repeat NOT, bid 1NT if you can bid 1H or 1S. If you bid 1NT after partner opens 1D, but you had 4 Hearts, later in the bidding, when you might need partner to know you have them, partner will never believe you. The net effect of this is to damage partnership confidence. If partner had 19 points, 6 Diamond and 4 Hearts, you're NEVER going to find the Heart game and you miss out on the lovely game /rubber bonus. More importantly, your partner will miss the bonus and will not be a happy bunny. Still inwardly cross, they will probably make a mistake on the next hand, and now the partnership is in a downward spiral. All because you didn't bid 1H when you could.
2) You can give partner a weak raise with 4 card support (1H - 2H) but if partner opens a minor suit, the above constrictions apply: even with 4 card support for a minor, show a 4 card – or better – major FIRST.
Responding with 10 - 12 HCP
The "invitational" range: i.e. you can suggest game to partner but you cannot yet insist on it.
1) Bid a 4 card major at the one level (I do go on don't I?)
2) You can bid a new suit at the 2 level. It can be 4 cards long [except!!: After 1S, 2H's promises 5 hearts]
3) You can make a “limit” raise of partner's suit (1H - 3H) with 4-card support. If partner opens a minor though, if you remember, bid a 4 card Major first…
4) There used in Old fashioned Steam Age Acol to be a bid that covered this range and it was 2NT. This bid has almost completely dropped out of usage and you should forget you ever learnt it. The reason is that modern computer based study of percentage actions has shown that it is correct to open on fairly weakish hands that would have horrified our forefathers. Consequently the best place to play may be in a suit at the 2 level. Impossible if you bid 2 NT.
Responding with 13+ HCP
The Game range
1) Make sure of game.
2) Bid a 4 card major at the 1 level if you can (again already?)
3) Er, that's it.
More next week