A bid of 2C by a passed hand, in response to an opening one of a major,
is conventional, showing 3 card support and values for a limit raise of partner's suit..
Responder Bids 2D to SHow 4 card support and values for a limit raise.
In either case, opener's rebid of the major shows a weak hand, while any other bid shows standard opening values or better.
E.g.
You
Q109
74
A9643
KJ4
You Opp Ptr Opp
P P 1S P
2C P 2S P/O
You bid 2C to show a limit raise with exactly three card support and partner rebid 2S to show a weak hand.
You Opp Ptr Opp
P P 1S P
2C P 2D P
4D P 4S
Partner has an average opening hand, and the 2D bid is a game try. I made a jump raise to show excellent support, in case partner
was considering a possible slam try. The 4S signed off in game. I'd expect partner to hold something like
AJxxx
Qxx
Kxxx
Ax
2D response.
Q1098
9
AJ64
K863
You Opp Ptr Opp
P P 1S P
2D P 2S P
3C P 3S
2D showed a limit raise with four card support. Opener stopped at 2S to show a weak hand. Because of the decent shape you might want to make a game try. After all, if partner holds
something like
KJxxx
xxx
x
AQxx
Game would be on. Ptr's rebid shows no support in clubs and no interest in game.
If you think this hand is too good for a limit raise because of the side shortness, I would
suggest you either (a) use a jump raise to show four card support with side shortness, (b) use a non-forcing variation of the Under Jump Shift convention.
Personally, I'm a fan of UJS. However, I would suggest that you don't get too excited by this hand. Switch opener's holding to
AKJxx
QJx
xx
xxx
and 3S should be an adventure.
Also, if you're slightly insane like me, you might open a hand far weaker than this example, where 3S is a strong candidate for a penalty double.
Competitition.
Most partnership play this convention in competition, though other conventions are also available for these hands as well.
I'd strongly suggest you work out the details of this convention with your partner to avoid miscommunication.