~Responding to Partner’s Overcall

An overcall interferes with your opponents’ bidding, invites partner into the auction, and helps your partner with the opening lead and the defense.
Responding with support
    When partner overcalls and you have 3-card support, you must decide how to proceed. Raising partner’s suit is your first priority. Here are two options on how to proceed:
Option #1: with 3-4 trumps
~Make a simple raise with 6-10 “support points.” Support points include high-card points plus points added for distribution.
~With 11+ support points, cuebid the suit the opponent opened.
    ~With a weak overcall, the overcaller will rebid the trump suit at the lowest level.
    ~With a minimum opening hand, the overcaller will jump in the trump suit to invite or play at game, whichever is appropriate.
    ~With a few extras, the overcaller will bid game.
    ~With more than a few extras, the overcaller will invite slam.
~With 4-6 support points and four or more trumps, make a preemptive jump raise.
Option #2: with 3-4 trumps
    ~Make a simple raise with 6-8 support points and 3-4 trumps; make a jump raise with 8-10 support points with 3 trumps with an honor or any four trumps. The quality of the hand, the relative vulnerability, and the quality of the trump suit will dictate which bid you make.
    ~With 11+ support points, cuebid the opponent’s suit and proceed as in option #1.
    ~With 4-6 support points and four trumps, let the quality of the hand and vulnerability determine whether you pass, make a simple raise, or make a jump raise.
Example #1:
You hold: S K52    H T65    D AT42    C J43 and the bidding has gone
West    North    East    South
1D       1S         P         ?
Bid 2S. Although you have a good 8 HCP, you have only three trumps and no ruffing values. Supporting partner is more important than showing your diamond stopper with a 1NT bid. If the opponents persist in diamonds, you are happy to defend.
Example #2:
You hold: S K642    H KT6    D A942   C 43 and the bidding has gone
West    North    East    South
1D       1S         P         ?
Bid 2D to show a limit raise or better in spades with 3+ trumps.
Example #3: 
You hold: S K742    H AK4    D A4    C J432 and the bidding has gone
West    North    East    South
1D       1S         P         ?
Again, bid 2D. If partner has made a weak overcall, he will bid 2S and you will bid 4S. If partner bids 4S, showing a full opener or better, you may choose to bid 4NT as a slam try.
Example #4: 
You hold: S KT42    H 42    D T4    C    Q7542 and the bidding has gone
West    North    East    South
1D       1S         P         ?
The option 1 bid would be 3S, preemptive with a weak hand and four trumps; with unfavorable vulnerability, the better bid might be 2S. The Law of Total Tricks supports a 3S bid regardless of vulnerability.
Rebidding after overcalling
After you overcall, partner should let you know if he has support. Your overcall may be based on a wide range of strength. Therefore, after a cuebid, you have to tell partner how strong you are. If you are strong to make game opposite a limit raise, you should bid it. After you bid game, partner with extras may make a slam try. If you made a weak overcall, you should rebid your suit at the cheapest level; if partner has extras, he will either invite game or bid game. If you have a hand that is in between, you should make a bid in a new suit, even one with three cards. If a new suit bid is not available at the two-level, you can make a Help Suit Game Try by bidding a new suit in which you need help to bid game. 
    Here are some examples. In each case, the bidding has proceeded
West    North    East    South
                        1C       1S
P          2C        P          ?
Example #1: You hold
S AQT43    H 43    D K84    C T53   
Bid 2S. Your hand is too weak to make game if partner has only limit raise strength. With extras, partner will make a game try or just bid game.
Example #2: You hold
S AQT43    H 73    D AK4    C KJT
 Bid 4S. You are just a bit too weak to make a slam try opposite a limit raise. If partner has extras, he will make a slam try.
Example #3: You hold
S AQT43    H 53    A84    C KT4
Bid 2D. This shows an intermediate hand that is too weak to bid game opposite a limit raise but strong enough to invite game if partner has a bit extra.
Responses when you don’t have support
Often you will not have support for partner’s suit. If you have a stopper in the opponent’s suit, you can bid 1NT with about 6-10 HCP. If you have a good suit of your own, you can bid it.
    Here are two examples. In each case, the bidding has proceeded:
West    North    East    South
1H       1S         P         ?
Example #1: You hold
S J3    H KJ43    D KT3    C JT84
Bid 1NT. You have a heart stopper and a balanced hand with 9 HCP and only two card support for partner’s suit.
Example #2: You hold
S 83    H KQ3    D J3    C KQ9842
Bid 2C. You have 11 HCP and a good suit.
    In both examples, you have enough values to act. If partner rebids 2S, you can pass; if partner jumps to 3S, you can raise to game.