Contract bridge, usually known simply as bridge, is a trick-taking card game of skill and chance (the relative proportions depend on the variant played). It is played by four players who form two partnerships (sides); the partners sit opposite each other at a table. The game consists of the auction (often called bidding) and play, after which the hand is scored.

The bidding ends with a contract, which is a declaration by one partnership that their side shall take at least a stated number of tricks, with specified suit as trump or without trumps. The rules of play are similar to other trick-taking games with the addition of the fact that one player's hand is displayed face up on the table as the "dummy".

Much of bridge's popularity owes to the possibility that it can be played in tournaments of theoretically unlimited number of players; this form is referred to as duplicate bridge. Competitions in duplicate bridge range from everyday ones in numerous small clubs to World Championships and Olympiads.

Game play
Two partnerships of two players each are needed to play bridge. The four players sit around a table with partners opposite one another. The compass directions are often used to refer to the four players, aligned with their seating pattern. Thus, South and North form one partnership and East and West form the other.

A session of bridge consists of several deals (also called hands or boards). A hand is dealt, the bidding (or auction) proceeds to a conclusion and then the hand is played. Finally the hand's result is scored.

The goal of a single deal is to achieve the highest score with given cards. The score is affected by two principal factors: the number of tricks bid in the auction, and the number of tricks taken during play. Broadly speaking, there is an incentive to accurately bid to the optimum contract and then to play to make the contracted number of tricks (or more if good play or luck allows). Thus, in the bidding stage, the pairs compete to see who proposes the highest number of tricks, and the side which wins the bidding must then fulfill that bargain by taking at least the contracted quantity of tricks in play to obtain a score. The number of tricks bid and the trump suit (or lack thereof) are referred to as a contract. The trump suit, or its absence (no trumps) is referred to as denomination or strain. If the side which wins the auction then takes the contracted number of tricks (or more), it is said to have fulfilled the contract and is awarded a score; otherwise, the contract is said to be defeated and points are awarded to the defenders.

In finding an optimum contract, it can sometimes pay to bid slightly too high (or bluff), and lose a small number of points, rather than allow the opposing side to bid and make a larger score. This is known as a sacrifice, and is quite common if both sides are contesting the final contract. This aspect is more common in duplicate bridge (as played in competitions and many clubs) where the goal is to get a better score than any other partnership facing the same hands, by however small a margin and in whatever way possible.

Dealing
The game is played with a deck of 52 cards. On each game, one player is the dealer, who distributes the cards and also bids first. The dealer changes on each game, usually going clockwise around the table.

In rubber bridge (or other non-duplicate games), the cards are shuffled before every game, and the dealer distributes all the cards clockwise one at a time, starting with his left-hand opponent and ending with himself, so each player receives a hand of thirteen cards. At the same time, for convenience, the dealer's partner usually shuffles a second deck, to be ready for use on the following deal. The dealer's left-hand opponent will deal next. Each hand in rubber bridge is therefore random and unrelated to other hands played, and a lot of the time the score depends on the cards as well as the skill of play.

In duplicate bridge, the hands are shuffled only once, at the beginning of the session, and dealt into the same four hands of 13 cards, and these deals are preserved for the entirety of the tournament. In this way, each time they are played, the results for different players will be comparable and any element of chance due to some players having better cards is eliminated. Cards in duplicate are passed around from table to table in bridge boards, plastic containers which clearly mark the hands, identifies who holds which cards, and also contains a folded slip of paper to record each teams' scores after the game. In some competitions, boards are pre-dealt prior to the competition, especially if the same hands are to be played at multiple locations (for example in a large national or international tournament). There are also special machines for pre-dealing on large tournaments. At each subsequent table, the four players pull their cards from the board and count them to ensure that there are 13 cards in their hand.

Unlike rubber bridge, in duplicate games players do not put their cards in the middle of the table as usual; instead played cards are placed immediately in front of each player and turned face down once each trick has been completed. This is to allow each player to return his hand, intact, to the board after he has finished it, so subsequent tables can play the same deal, unaltered. It also means that in case of a query or other irregularity during a competition, it is clear exactly who played which cards, and the order in which they were played.

The auction determines the declaring side and the final contract, which consists of the denomination and level (number of tricks). Only one of partners of the declaring side, referred to as declarer, will play the hand, while the other will become the dummy (i.e. doing nothing). In addition, if the final contract is doubled (by the opponents) or redoubled (by the contracting partnership) the scoring of the hand is increased, whether the contract is made or defeated.

During the auction, each player makes a call at his turn, which must be one of the following:
Bid (stating a level and a denomination)
Double (when the last call other than pass was a bid by an opponent)
Redouble (when the last call other than pass was a double by an opponent)
Pass (when unwilling to make one of the three preceding calls, i.e. "abstain")
(Note: although technically incorrect, the word "bid" is also often used informally in place of "call")

The auction starts with the dealer and proceeds clockwise with each player, having first evaluated their hands, making a call in order. The auction ends when 3 successive passes occur after a bid, double or redouble (or if all 4 players pass in the first round).

A bid specifies how many tricks the bidder believes that the partnership can take using both their hands, and with which strain as trump. Any bid starts with the assumption that the bidder can make at least six tricks, called book, plus the stated number of additional tricks. So the bid includes a level (from one to seven, representing how many tricks beyond six the bidder proposes to make) and a denomination (also called strain), which is either a suit or "no trump". For instance, "3 hearts" asserts that his partnership can take nine tricks (book plus three) with hearts as the trump suit. There are 35 possible bids, which include all combinations of 5 denominations and 7 levels (7-13 tricks). Thus to bid, a player is effectively stating they expect their partnership to win at least 50% of the available tricks in the pack (50% x 13 tricks = 6.5 tricks) if allowed to play the named contract.