A double saying that you do not expect your opponents to make their contract is called a penalty double. It increases your score if the contract fails and also increases their score if the contract makes.
The other type of double is a takeout double which is used to ask partner to bid.
How to tell if double is penalties or takeout
It can be hard to tell the difference! Some things to keep in mind:
- Doubles of low level contracts in which the opposition have shown a trump fit are takeout.
- Doubles of notrump bids are penalties.
Doubles of notrump bids.
If your side has the majority of the points then doubling their 1NT contract can be more profitable than playing the hand in a contract your way.