Duplicate bridge is the most widely used form of bridge used in tournament settings, and is also played in many bridge clubs. It is called "duplicate" because the same bridge hand is played multiple times, using boards to keep and pass each player's hand intact. Final scores are calculated by comparing your results with others who played the same hand.
The overall USA organizations are:
In duplicate bridge, a player normally plays with the same partner throughout an event. The two are known as a "pair". There are two exceptions: on team events with five or six members swapping partners for portions of the event, and in individual tournaments, in which players change partners for each round.
Matchpoint Pairs Game
The most common form of pairs game is the matchpoint pairs game.
In a matchpoint pairs game, each deal is played a number of times, after which all the scores are compared. (See Contract bridge for the scoring method for duplicate bridge.) Each partnership scores 2 matchpoints for each other partnership that scored fewer points with the same cards, and 1 point for each partnership that scored the same number of points. These matchpoints are added to determine the winner. Scores are usually given as percentages of a theoretical maximum: 100% would mean that the partnership scored more than every other partnership on every single hand. Colloquially, scoring the maximum number of matchpoints on a certain board is known as a "top board", and scoring zero matchpoints is a "bottom board". The terms "high board" and "low board" are also used.
(Note: in the United States, scoring is 1 point for each pair beaten, and one half-point for each pair tied.)
Team Game
In a team game, two pairs normally constitute a team. (Teams of five or six members are sometimes permitted, but only four members play at any given time.) Two teams compete using two tables and having one pair from each team seated at each table, at opposite directions. (For example, team A may sit North-South at table 1 and East-West at table 2; then team B would sit East-West at table 1 and North-South at table 2.) A relatively larger number of boards are played (usually six to eight for "Swiss teams", usually 12, 24, or more for knockout events). The boards are moved (usually by a caddy) so that they are all eventually played at both tables.
Suppose Team A plays Team B. The first time a hand is played, one partnership from Team A takes the North-South cards and one partnership from Team B takes the East-West cards; when the hand is played again, it is played by the other two partnerships, but this time with Team A holding the East-West cards and Team B holding the North-South cards. Of course the teams may not discuss the deals between the two plays. After each deal has been played twice, the scores per deal are compared, and a score is given depending on the net total score from the two times the deal was played. For example, if one pair scores +1000 on a deal, and their teammates score -980, then the team's net score on that deal is +20.
One of two forms of scoring is then used to calculate the winner. At "board-a-match", each hand has equal weight; each hand is won, lost, or tied. At IMP scoring, the difference is converted using a scale of 0-24 IMPs (International Match Points) that compresses big differences in score.
IMP Pairs Game
IMP pairs games are played the same way as matchpoint pairs games, but the scoring uses the IMP tables to make big differences in the results at the table also yield big differences in the results. At matchpoints, making one more overtrick than everybody else on a board gives the same result as making a slam that nobody else bid, whereas at IMP scoring, the slam is given more points.
There are at least two methods of using the IMP tables to score a board in a pairs event. One is to take the IMPs against the average score on the board, excluding the top and bottom result. This is often called Butler Pairs scoring. Another is to take the IMPs difference against each other play of the board, or Cross-Field IMP Pairs.
IMP Tables
There are many different IMP tables. A sample 24-point IMP table is:
Point IMPs difference 0 - 10 0 20 - 40 1 50 - 80 2 90 - 120 3 130 - 160 4 170 - 210 5 220 - 260 6 270 - 310 7 320 - 360 8 370 - 420 9 430 - 490 10 500 - 590 11 600 - 740 12 750 - 890 13 900 - 1090 14 1100 - 1290 15 1300 - 1490 16 1500 - 1740 17 1750 - 1990 18 2000 - 2240 19 2250 - 2490 20 2500 - 2990 21 3000 - 3490 22 3500 - 3990 23 4000 or more 24
Individual Events
An individual event in duplicate bridge is one where each round a player is paired with a different partner. Scoring is usually using matchpoint pairs scoring, but IMP pairs scoring can be used. There are various methods for assigning partners. In some methods, a given set of players always sit North, another set sit South, another set sit East, and a final set always sit West. This can be used to ensure that each pair consits of a realatively experienced or skilled player, adn a relatively inexpereinced player.
Individual events are more complex to run, and require that the players get accustomed to new partners on a frequent basis. For those reasons they are less popular and less common than pair or team events, but some players are very fond of them.
Contrast with Rubber Bridge
Duplicate bridge differs from rubber bridge: whereas the goal in rubber bridge is to win more points than the pair of people you are playing against, in duplicate bridge the goal is to win by a larger margin (or lose by a lesser margin) than all the other pairs playing in the same direction as you. Because of this, strategies are different. In rubber, 30 points above the line for an overtrick is unimportant. In match-points duplicate, it is common for those 30 points to mean you get a top score instead of average. In rubber, an occasional 800-point penalty is disastrous, but in duplicate it is no worse than any other bottom score. International match points is in the middle of these extremes. Huge penalties are worse than small penalties, but then 30 point differences are only moderately important.