Five-card majors

Five card majors is a contract bridge convention which is very powerful and standard in modern bidding systems today [http://www.acbl.org/learn/index.php "Learn to Play Bridge" Software from the ACBL] , including Standard American and Bridge Base Basic. Partnerships who agree to play five-card majors will usually only open the bidding in a major suit with at least 5 cards in that suit and with at least 13 points.

The concept

Typically when a bridge player makes a natural bid, he is promising at least 4 cards in that suit and asking partner if it will be an advantageous trump suit for the partnership. Because of the power of naming a trump suit with an 8-card fit, the responder with 4 more cards of that suit will support his partners bid as if to say "we have found our 8-card fit."

With 13 cards in each suit, an 8-card fit implies that only 5 trump cards can be held by the opponents. They will most likely be distributed 3-2 or 2-3 among the opponents, so playing trump for three rounds will probably draw all trump cards from the opponents and leave 2 additional trump to be used separately for offensive purposes. However if the trump cards break 4-1 or 1-4, then drawing trump will result in no trumps left for offensive purposes.

The value of 5-card majors can be understood then on two levels:
*When partnerships have a 5-3 distribution in a major suit (hearts or spades), then the 8-card fit in a major suit is easier to find for the player with only 3 cards in the major suit. The strong preference to play Duplicate bridge in the major suits at the game level makes the 5-card major convention very attractive.
*When the trump suit can be declared with a 5-3 fit, then often 1 extra trick can be taken due to the extra trump card in declarer's hand because
**if the opponents 5 trump cards are distributed 3-2 or 2-3, then declarer will have two remaining trump cards to use in continuing play.
**if the opponents 5 trump cards are distributed 4-1 or 1-4, then declarer can pull trump for 4 rounds and still have 1 trump card in declarers hand for continuing play.

To play 5-card majors

Both partners must agree to follow the 5-card major bidding convention on their opening bid. Opener must have at least 5 cards in hearts or spades to start the bidding with that suit. Responder is expected to show support with 3-card support, indicating an 8-card fit.With only 4 cards in a major suit, the opening bidder is expected to open one in the best minor suit (which may show as few as 3 cards in the minor suit bid).After the opening bid, the 5-card limitation is no longer in effect and any other bid typically promises only 4 cards as before.Bridge partnerships who use five-card majors also typically use the "short 1Clubs" opening bid (opening one club does not promise 4Clubs) to indicate that opener has:
*at least 13 points and interest in winning the bid,
*no 5-card major (else opener would have bid it, unless also holding a 6-card or longer minor),
*no 4 diamonds (else opener would have bid 1Diams),
*no 4-4 distribution in the majors (else opener would have bid 1Diams)

References