After the Opponents Overcall

(Adapted from Larry Cohen:

https://www.larryco.com/bridge-articles/after-the-opponents-overcall )

Let's start on the 1-level.

The opponents overcall partner's 1-level opening bid with a suit bid.

WEST	NORTH	EAST	SOUTH
	1(something)	1(something)	?

If partner opens 1-of-a-suit and the opponents overcall in a suit:

- Doubles are negative.

- New suits by responder are forcing one round (if on the 2-level, 10+ points).

Examples:

1D (1H) 1S = forcing one round (6+ points)

1D (1S) 2C = forcing one round and 10+ points

-- New suits by responder promise 5 or more cards, except when the opponents overcall 1C with 1D and we respond 1H or 1S.
Examples:

1C (1D) 1 MAJ = 4+ in that major
1D (1H) 1S = 5 or more spades and 1D (1S) 2C = 5 or more clubs.

-- New suits by responder on the 3-level (not a jump, though) are forcing to game. Example: 1S  (2D)  3C = 5+ C, forcing to game.

-- New suit jumps by responder is weak (weak jump shift in competition) Example: 1D  (1S)  3C

- Raises

-- Single raises are natural and mean what they would have meant without the interference. If forced to raise to the 3-level, it shows a little extra. 

Example:  1H (2S) 3H = 3+ H and 8-10 in support (approximate)

-- Cue-bidding the opponent's suit shows a limit-raise or better.

Example: 1S (2D) 3D

-- Jump raises are preemptive.  Example: 1S (2D) 3S

- Notrump bids are natural (1NT=6-10, 2NT=11-12, 3NT=13-15) with a stopper in their suit (and balanced).
Example:  1D  (1S) 1NT=6-10 with spades stopped

More examples:

OPENER	OPPONENT	RESPONDER	MEANING
1C	1D	1 MAJ	4+ cards 6+ points
1C	1H	1S	5+spades, 6+ points
1C	1S	2D	5+D 10+points
1C	2S	3D	5+D GF
1C	1H	2S	Weak - preemptive
1D	1H	2H	4+D, Limit Raise+
1D	1S	2NT	11-12 balanced NF, S stopper
1D	1S	3NT	13-15 balanced, S stopper
1S	2H	3S	4 S, weakish (0-7)
1S	2H	3H	3+ S, Inv+

The opponents overcall with 1NT:

If they overcall 1NT, assuming no conventions, a double is penalty, and a new suit bid is natural, NF

Examples:

WEST	NORTH	EAST	MEANING
1C	1NT	Double	Penalty
1S	1NT	3S	Preemptive
1H	1NT	2D	6+D, NF

The opponents overcall with preempt: 

A new suit by responder is forcing for one round on the 2-level. If responder bids a new suit on the 3-level, it is game forcing. 

The opponents overcall our preempts:

- Doubles are penalty.

- Raises are competitive/further preempting--not inviting.

- 2NT (when possible, of course) is Ogust

For example:

WEST	NORTH	EAST	MEANING
2H	2S	Double	PENALTY
2H	3	3H	NOT INVITING 4H
2H	2S	2NT	Ogust