If the opposing pair have opened the bidding in a game of bridge you have to decide whether to pass and let them continue to make the contract or whether to make a bid for your partnership.
Why would you want to overcall?
1 of a suit.
In order to make a suit overcall you should have 5+ cards in the suit and usually at least two honour cards.
If you want to bid at the one level (for example 1 heart) you need 8 – 15 points.
If you need to bid at the 2 level you should hold 11- 15 points.
Suit Quality Test
If you hold minimum points for overcalling (8 – 10 at the one level or 11 – 13 at the two level) you need to decide if your hand is suitable.. You can do this by using the "Suit Quality Test".
Add the number of cards in the suit you want to bid to the number of honour cards held in that suit. If the number is at least equal to the number of tricks you want to bid then go ahead.
If you do not have sufficient suit quality do not overcall.
If you wish to bid at the one level that means you are bidding for 7 tricks (6 + 1). A hand that has 5 cards in the suit plus 2 honours card would score 7 in the suit quality test and therefore be suitable for overcalling at the one level.
If you wish to overcall at the two level you are bidding for 8 tricks (6 + 2). A hand that has 5 cards in a suit plus 3 honours, or 6 cards plus 2 honours would score 8 in the suit quality test and therefore be suitable for overcalling at the two level.
If you have 12 -15 points AND a 6+ suit card you can make a jump overcall. For example if the bidding has opened with 1 diamond and you have good points and cards in hearts a bid of 1 heart would be available – but if you have enough points and cards you can jump to a bid of 2 hearts.
If you have 6-9 points AND 7+ cards in a suit you can make a pre-emptive overcall. This is a "double jump". Do this if the points in your hand are mostly contained in the 7 + card suit. If you have aces and/or kings in other suits you might be better of defending rather than overcalling.
If you have 16+ points don't make a suit overcall. Depending on the make-up of your hand you should either make a 1NT overcall or double.