[navit]

In general how do I respond to partner's opening 1 major bid if I have 12+ points and 3 card support but no 5 card suit. The same question with 4 card support. Opponents passed.

[gwnn]

Bid 2♣ with 3 card support, 2NT with 4 card support.

[aguahombre]

Loaded question. If it is "how do you respond to.....", it is still loaded.

SAYC or 2/!? Have you agreed to any fit-showing conventions? In your style, is 12 enough for insisting on game with 3-card support? This question is best answered by you and your partner after agreeing what system you play.

[awm]

One can play special conventions to handle these hand types of course. One that is common is Jacoby 2NT, where jumping to 2NT shows a game-forcing hand with four-card support for partner's major. Certainly there are other (less common, but perhaps superior) options available.

Without such conventions, the thing to do is to make a 2/1 bid in a minor and then plan to raise partner's major at the next turn. There is some debate about which minor to bid. The two main schools of thought on this are: (1) Always bid clubs, so that 2♦ shows five and the 2♣ bid becomes "clubs or balanced" -- this helps substantially when you do have five diamonds because partner will know it is five, and at least the 2♣ bid is very cheap and allows space to sort things out (2) Bid the longer/better minor; here both minors are frequently four (and perhaps very occasionally three) but partner can assume you at least have values in the suit you bid and evaluate his hand accordingly.

[Free]

You can do all sorts of stuff. Some use a very basic approach, many use 2♣ as some sort of waiting bid to see what opener has, many use 2NT as 4+ cards and some HCP range (inv/inv+/GF).

I've been playing something like 1♥-2NT (after 1♠ we bid 3♣ to show this hand) as 10-13 balanced with 3+M, with 14+HCP we respond 2♣ (for us a relay). Advantage is that opener can place the contract after a 10-13 most of the time, without revealing information to opps.