Two commonly-played conventions over a NT opening or overcall are Stayman and Jacoby transfers. They both do not need to be alerted.
Stayman
Stayman asks the NT player for 4-card majors, in hopes to play in a 4-4 major fit. After a 1NT opening or overcall, 2♣ is Stayman. After a 2NT opening showing 20-21 points, 3♣ is Stayman. The responder needs to have at least 8 points and a four card major to ask the NT bidder for their four card major.
In response to the 2♣ bid, the 1NT opener would bid one of the following:
2♦: denying a four-card major
2♥: 4 hearts
2♠: 4 spades
Depending on what the opener showed, the responder can either invite to game or go to game themselves. If you have 8-9 points, you should invite to game. The NT opener should accept the invite and go to game if they have top of their range (i.e 16-17 points with 1NT opening and overcall). Bid one of the following invitational bids as a responder with 8-9 points:
2NT: telling the opener that you don’t have the major they bid/no major fit anywhere
3M (the 4-card major the opener showed): telling the opener that the major they bid was the suit you have a 4-card major in; supporting and showing a 4-4 fit
If you have 10+ points as the responder, you can bid:
3NT: no 8-card fit in the major opener showed 4 cards in
4M (the major opener showed): 4-4 fit
Let’s walk through what Stayman should look like from one of the CK Team Games.
Here, the 1NT overcall by North shows 15-18 points, balanced with a stopper in the opponent’s suit. Systems are still on. South bids 2♣, which should be Stayman, asking partner if they have a four card major while S has a four card major of their own. South can’t bid Stayman here— remember you need 8+ points and a four card major. South here only has 2 3-card majors and 7 points, not enough.
Let’s pretend South does have a 4-card major and 8+ points, and North responded 2♦. 2♦ should mean that North does not have any 4-card major. Since North has a 4-card major on this board, North should’ve bid 2♥. After that, depending on South’s hand, South would either go straight to game with 10+ points or invite to game (2NT no heart fit or 3♥).
Jacoby Transfers
Jacoby transfers are transfers into majors made by the responder. The responder would bid one suit lower than their intended 5-card major. The following bids are transfers, asking the 1NT opener to bid the next higher suit above their bid:
2♦: asking opener to bid 2♥, responder must have 5 hearts themselves, 0+ points
2♥: asking opener to bid 2♠, responder must have 5 spades themselves, 0+ points
After the opener transfers accordingly, the responder should bid one of the following:
Pass: less than 8 points, just want to play in a suit contract rather than NT
2NT: 8-9 points, inviting to game, asking partner if they have 3 cards in the transferred suit. We bid NT even after the transfer because responder doesn’t know whether or not opener has 3+ card support for them.
3M: 8-9 points, inviting to game, showing 6+ cards in the transferred major suit
3NT: 10+ points, only 5 cards in the transferred major
4M: 10+ points, 6+ cards in the transferred major (opener shouldn’t have any singletons, so there is a guaranteed 8-card fit).
Depending on the hand, the opener can either go to game (3NT or 4M) with top of range (16-17 points), sign off at 3M after responder’s 2NT invite showing support but not top of range, or pass.
Here’s an example from a CK game:
After the 1NT opening by North, South bids 2♦, which is a transfer to hearts. South can transfer here because S has a five card major, hearts. North correctly bids hearts for South, completing the transfer. Now after the transfer, South should bid 2NT, inviting North to game (8-9 points) either in hearts (if N has 3+ hearts) or NT.
Since North has 4 strong heart support and top of the 15-17 range, after South’s 2NT, North should jump straight to 4♥.