When your opponents open the bidding first your requirements for bidding are affected by the knowledge that they have opening values. We need to consider a little arithmetic. Opener has at least 12 high card points (HCP) and if you have a hand of approximate opening strength you have another 12. Therefore the two partners have at most 16 HCP between them, possibly considerably less! So how are they split? There are three possibilities (P), which will be extensively referred back to later:
P1) Your partner has most of them. Therefore you should play in game.
P2) Opener’s partner has most of them. Therefore they will outbid you, probably to game.
P3) They are evenly split. It is probable that both sides can make a part score at the two or three level if their chosen suit is trumps, It is even possible, though less likely, that both sides can make game.
Your overcalling strategy must try to cater for all three. You must convey to partner assurance of some HCP so he can identify P1). You should try to help the defence if P2) applies by suggesting a good opening lead. You must try to contest the part score if P3) applies.
Many decisions are not clear cut so how do you decide? Broadly speaking you are likely to bid on a hand that wouldn’t have merited an opening bid if you have a well shaped playing hand. On the other hand you might pass on a hand that would have merited an opening bid if you feel that your hand is as useful for defence as for playing.
A suit overcall if opponents open a suit
These criteria assume nobody is vulnerable. If only your side is vulnerable be a bit more cautious, remember your overcall might be doubled for penalties. Conversely if only your opponents are vulnerable you can be more aggressive.
You need to understand the concept of suit quality. To find the suit quality of your suit count the number of cards in the suit and add the number of honours (but only count the 10 or Jack if you have a higher honour in the suit). The resulting number is the suit quality.
A suit overcall at the one level (e.g. 1♥ 1♠) shows 8-16 HCP and at least a five card suit. If you have only 8 -10 HCP, your suit should be very strong. If you only have 8-10 HCP your suit should have a suit quality of at least 7.
A jump suit overcall (e.g. 1♥ 2♠ or 1♥ 3♣ shows 12-16 HCP and a very
strong six card suit.
A suit overcall with a double jump is pre-emptive.
Now consider these hands if your right hand opponent (RHO) opens 1♥ with neither side vulnerable.
a) Overcall 1♠. You are particularly concerned about P2) above. If opponents win the contract and partner has to lead you are keen to guide him towards a spade! Note that there is a considerable difference between your playing strength and your defensive strength. If spades are trumps you probably have 5 tricks in your hand. If they play in hearts even your ♠A may be trumped! Your suit quality is 8
b) Pass. 9 HCP but suit quality only 6.
c) 1♠. This is necessary because of P3) although it is not very helpful if it persuades partner to lead a spade, This is a good example of a hand on which you must bid the lesser of evils!
d) Pass. This looks like a), indeed you have the same quality suit with identical HCP, however you need a bit more for an overcall at the two level and the risk of being doubled is that much greater.
e) 2♣. A minimum two level overcall. With these HCP and only a five card suit it would be wiser to pass. Your suit quality is 9.
f) 2♠. A minimum jump overcall.