For the list of conventions, see Category:Conventions.
In natural bidding systems, a call normally specify the meaning on the call (e.g. a 1♥ means that the caller wants to play hearts, an X means that the caller wants to penalize the opponents for not making their contract). However, even in natural bidding systems sometimes need to use artificial bids in order to pass information that cannot be described using natural bids. These bidding are called conventions. A bid, regardless if it is natural or artificial, will become the final contract after 3 consecutive passes. Therefore, the partner must respond to most conventional bid if the RHO passes.
- The sequence of 1♥ - (/) - 4NT normally specifies a Blackwood convention (or its variants). The 4NT bid does not say that the responder has a balanced hand, but instead asking the opener about the number of aces in order to avoid bad slams.
- An X after an opponent's 1 or 2-level opening normally does not mean penalty, but instead tells the partner I have a strong hand and short in opponent's suit, please choose a suit for me. This is called takeout double and is technically a convention. As this practice has existed in the very early stage of development and is adopted universally, this is usually considered as a part of natural bidding and an alert is not needed. Although the caller does not mean penalty, the partner can pass it to convert it to a penalty double.