A brief summary of common agreements. Google for more information.
Starting with the easy stuff and getting more complicated further down the page ...
This is a bid of 2C after partner has opened 1NT. It doesn't need to be alerted as it is considered standard.
2C asks Opener to tell Responder if they have a 4-card Major. If they have 4 (or 5) hearts then they respond 2H. If they have 4 (or 5) spades then they respond 2S. If they have both then they respond 2H.
If Responder doesn't have a 4-card major suit, then they respond 2D.
Responder will then make another bid, usually raising a major with a fit, or returning to NT without a fit.
Stayman can also be played after a 2NT opening bid, 3C, in which case it needs to be alerted.
A transfer is a bid of 2D or 2H after partner has opened 1NT. It should be alerted by Opener.
2D is a bid that is made with 5 or more hearts. It can be made with any strength of Responder hand.
2H is a bid that is made with 5 or more spades. It can be made with any strength of Responder hand.
Opener will normally complete the transfer by bidding the next suit up, the suit that Responder has !
Responder then gets a second bid. They will Pass with a weak hand, bid 2NT with 10-12 points, or 3NT with 13+ points if they have a 5-card major suit. Opener will correct back to the major suit if they have 3 or more.
If Responder has a 6-card major then they will rebid the major suit.
Super-Accept of a transfer
If Opener has a matching 4-card major then they can show this by breaking the transfer.
2NT = 4333 and 14 points
3 of the Major = 4333 and 12 or 13 points
a new suit = 14 points and an ace or king in the new suit
These should all be alerted.
Playing transfers, the 2S response to a 1NT opening bid is now available for something else.
A simple version is to use 2S for a balanced 11 points, and then make 2NT a balanced 12 points.
The more complicated version is to use 2S for 11+ points. Opener replies 2NT with a minimum (12 or poor 13), or bids a new suit at the 3-level with a maximum (good 13 or 14).
2NT is then free for something else ! We use it to bid a long minor and weak points. Opener must rebid 3C, and Responder passes with clubs, or corrects to 3D with diamonds.
The first time that you can't follow suit you make a discard.
You can use this moment to give partner a signal about your hand (and at other times).
Low = Like High = Hate knowns as "reverse attitude signals"
Play a low spot card from a suit that you would like partner to lead. 2,3,4,5
Play a high spot card from a suit that you don't want partner to lead. 7,8,9,10
Basically you play a low card in a trump contract, with high honours, to cash some quick tricks.
And against a No-Trump contract you discard your rubbish suits, but start with a high card. This tells partner to look elsewhere for tricks, and that you can't guard this suit.
You can also use this signal when partner leads a high honour, showing whether to continue the suit because you have the Q or J, or maybe if you are short and will be trumping.
This is a nifty little convention after partner opens a weak 2D or 2H or 2S.
Responder has an opening hand, 2 or 3 or 4 of partner's suit, and isn't sure whether to go to Game or not. Rather than inviting Opener to go to Game, or just blasting Game, Responder asks Opener about the quality of their hand and the weak two suit. They do this by bidding 2NT, which is alerted.
Opener replies based on whther they are 6/7 points minimum, or 8-10 points maximum; and also show the quality of their suit, does it have 2 of the top three honours AKQ ?
So, after 2NT :
3C = 6/7 points, only one top honour
3D = 6/7 points, two top honours
3H = 8-10 points, only one top honour
3S = 8-10 points, two top honours
3NT = 8-10 points, all three top honours, AKQ
Responder can then pick the final contract.
This also means that a single raise is not an invitation, just a pre-emptive raise.
What does it mean if you overcall the opposition suit with the same suit ??
e.g. East opens 1C and South bids 2C or West opens 1S and North bids 2S
MINOR SUIT : (1C) 2C or (1D) 2D
This shows both major suits, hearts and spades, at least 5 cards in both suits. 5-5 or better.
Partner just picks their longer major and bids 2H or 2S. With equal length pick the stronger suit.
MAJOR SUIT: (1H) 2H or (1S) 2S
A bit more complicated. This shows the other Major suit, and either clubs or diamonds. 5 of each suit.
If partner has 0 or 1 of the other Major then they will have something in both of the minors. They bid 2NT which asks partner to bid their long minor suit at the 3-level.
STRENGTH
You should have a few points to make these bids ! You don't always expect to make your contract but you can't afford to take a large penalty. I recommend 8+ points non-vulnerable, 10+ points vulnerable.
You should also have most of your points in the long suits, not Ks or Qs in the short suits.
(With 16+ points you can bid again.)
This is a similar bid using 2NT to show two 5-card suits. Partner just picks the best suit at the 3-level, so you need to be a little bit stronger than Michaels.
The bid shows the two lowest unbid suits.
After a major suit opening by the opposition, it shows both minors, 5-5 or better.
After a minor suit opening it shows hearts and the other minor, 5-5 or better.
This is a great convention to use when the opposition open 1NT.
X with 16+ points
2C shows both Majors, at least 5-4
Partner bids their longer Major, or 2D with equal length
2D is bid with a long Major suit, at least a 6-card suit
Partner bids 2H and you pass with hearts or correct to 2S with spades
2H shows 5 hearts and 4+ in a minor suit
Partner passes or bids 2NT to ask for the minor
2S shows 5 spades and 4+ in a minor suit
Partner passes or bids 2NT to ask for the minor
2NT shows both minor suits, 5-5 or better
All of these overcall bids should have 8+ points non-vulnerable, 10+ points vulnerable.
And most of the points in the long suits.
There are other similar systems to compete over 1NT including :
Cappelletti, Brozel, DONT, Notrab etc etc
Hopefully you're using 4NT to ask for aces:
5C=0 or 4 5D=1 5H=2 5S=3
RKCB is a more sophisticated response system.
Firstly, we also count the king of trumps as an 'ace'. Assume trumps is the last bid suit before the 4NT bid.
The four aces and the trump king are called 'keycards'.
And now we use the following responses:
5C = 1 or 4 keycards
5D = 0 or 3 keycards
5H = 2 keycards without the Queen of trumps
5S = 2 keycards with the Queen of trumps
You can usually tell whether it is 1/4 or 0/3 keycards from the previous bidding.
And now you also find out information on the top trumps.
Your partner opens 1NT and the next player doubles, showing 16+ points.
This is a dangerous situation and you should discuss an escape mechanism. There are many methods, below is a simple natural system, but discuss it with your partner.
If you have 10-12 points then you don't need to bid ! Partner should make 1NT and the score will be good.
1NTX making 7 tricks scores 180 points, better than any other part-score.
1NTX making 8 tricks scores 280 non-vulnerable, 380 vulnerable.
If you have 13+ points then you have a probable Game, and you score more by bidding and making Game.
If you have a good 8-9 points then you will probably make 1NT so Pass for now.
If you have less than 8 points then you probably won't make 1NT. You need to run.
Stayman and Transfers are OFF. If you have a long suit then just bid it, to play.
If you only have 4-card suits then you need to find a safe suit, maybe even a 4-3 fit.
Firstly Pass, and give partner the chance to bid a long suit themselves, probably a minor.
If Opener doesn't have a long suit then they Redouble, asking Responder to start bidding 4-card suits up-the-line, lowest ranked suit first. If a 4-3 fit is found then stop. If partner bids a suit where you have a doubleton then reply with your next lowest 4-card suit. Usually you find a good resting place.
If you have a balanced hand with more than 12-14 points then you should open 1-of-a-suit and rebid NoTrumps. This is a better way to describe the hand than rebidding a major suit.
e.g. you hold:
♠ KQ43 ♥ A97 ♦ K6 ♣ A852
You open 1C, partner responds 1D. Don't rebid 1S, rebid 1NT.
Now, partner may have four spades and you have just skipped spades to show your points.
Responder can use 2C as a bid to 'check back' to see if partner has 4 spades, (and/or 4 hearts). They are not going back to your clubs, there's no need, you are better playing in NoTrumps than clubs. Responder needs 8+ points to use Check-back Stayman, with 6 or 7 points just pass 1NT.
Opener then bids as if 2C was Stayman. 2H or 2S shows a skipped 4-card major suit; 2D denies a major.
If a fit is found then raise the major. If no fit then Responder goes back to NoTrumps.
It gets more complicated if Responder's first bid is a major suit.
1C - 1H - 1NT 1C - 1S - 1NT
1D - 1H - 1NT 1D - 1S - 1NT
Rebidding 1NT denies 4-card support for Responder's major suit, otherwise Opener would just raise.
So you can check back for the other major suit looking for a 4-4 fit; AND you can also check back in case Responder's major suit was a 5-card suit, looking for a 5-3 fit.
Check-back Stayman can also be used after a 2NT rebid, either as a jump showing 18-19 points, or after a 2-level response.
e.g.: 1C - 1D - 2NT
or 1S - 2C - 2NT (which shows 15-19 points).
3C is Check-back Stayman, asking Opener about the undisclosed length of major suits.