Five-card majors is a contract bridge

Contract bridge

Contract bridge, usually known simply as bridge, is a trick-taking card game using a standard deck of 52 playing cards played by four players in two competing partnerships with partners sitting opposite each other around a small table...

 bidding treatment which is very powerful and standard in modern bidding system

Bidding system

A bidding system in contract bridge is the set of agreements and understandings assigned to calls and sequences of calls used by a partnership, and includes a full description of the meaning of each treatment and convention...

s, including Standard American

Standard American

Standard American is a common bidding system for the game of bridge in the United States, also widely used in the rest of the world. This system, or a slight variant, is learned first by most beginners in the U.S. and may be referred to as 'Goren'; a dominant version used in on-line computer...

, Bridge Base Basic

Bridge Base Basic

Bridge base basic, also known as BBO basic, is a bidding system for the game of bridge based on Standard American Yellow Card. It is simplified, suitable for beginners, and widely used in internet bridge particularly on Bridge Base Online...

, and 2-over-1 game forcing

2/1 game forcing

2/1 game forcing is a bidding system in modern contract bridge in which, after a one-level opening bid, a non-jump response in a new suit at the two level commits the partnership to bidding at least game....

. Partnerships who agree to play 5-card majors will usually only open the bidding in a major suit with at least five cards in that suit and with at least 13 points.

The concept

Typically when a bridge player makes a natural bid in a major suit (hearts or spades), he is promising at least four 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 four or 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 five 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 two 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, the 8-card fit is easier for the player with the three cards to find. The strong preference to play duplicate bridge
  Duplicate bridge
  Duplicate bridge is the most widely used variation of contract bridge in club and tournament play. It is called duplicate because the same bridge deal is played at each table and scoring is based on relative performance...
  
   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 one extra trick can be taken due to the extra trump card in declarer's hand because
  • if the opponent's five trump cards are distributed 3-2 or 2-3, then declarer will have two remaining trump cards to use in continuing play.
  • if the opponent's five trump cards are distributed 4-1 or 1-4, then declarer can pull trump for four rounds and still have one trump card in declarer's hand for continuing play.

But 5-card majors have several drawbacks :

• Immediate 5-3 fits occur less frequently than immediate 4-4 fits (16.3% of the time versus 11.8%) reducing the probability of auctions such as 1-3
• Since playing 5-3 fits needs (at least) three turns to establish so there is often no trick gained by ruff in the short hand, while 4-4 fits can lend themselves to cross-ruff
  Ruff
  The Ruff is a medium-sized wading bird that breeds in marshes and wet meadows across northern Eurasia. This highly gregarious sandpiper is migratory and sometimes forms huge flocks in its winter grounds, which include southern and western Europe, Africa, southern Asia and Australia...
  
  . In the 5-3 case, the two remaining established cards (assuming the opponents cards are 3-2 or 2-3) can also bring tricks in no trumps, if there is an entry to the hand which owns these cards.
• Immediate discovering of 5-4 fits is possible on one way with 5-card majors, and on two ways with 4-card majors.
• Hands with 4-card majors and no 5-card major are opened by one of a minor suit
  Minor
  Minor means "not important", and in Latin "smaller". It may also refer to:* an underage child, see Minor * Academic minor, a secondary focus of a Bachelor's degree, the primary focus being an academic major-Mathematics:...
  
  , which is less informative and more easily preempt
  Preempt
  Preempt is a bid in contract bridge whose primary objectives are to thwart opponents ability to bid to their best contract, with some safety, and to fully describe one's hand to one's partner in a single bid. A preemptive bid is usually made by jumping, i.e. skipping one or more bidding levels...
  
  ed by opponents than a 4-card major opening.

To play 5-card majors

Both partners must agree to follow the 5-card major bidding treatment on their opening bid. Opener must have at least five 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 four cards in a major suit, the opening bidder is expected to open one in the best minor suit (which may show as few as two 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 four cards as before.
Bridge partnerships who use 5-card majors often use the short club, a 1 opening bid made on a 3-card club suit that indicates opener has:

• at least 13 points
  Hand evaluation
  In contract bridge, various bidding systems have been devised to enable partners to describe their hands to each other so that they may reach the optimum contract. Key to this process is that players evaluate and re-evaluate the trick-taking potential of their hands as the auction proceeds and...
  
  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-card diamond suit (else opener would have bid 1).

There is strong pressure upon responder to bid a 4-card major even after an intervening bid. 1-1NT-2 as first response shows partner has four hearts. Opening bidder will not raise the 2 bid with only three hearts.