A bishop is a piece in the strategy board game of chess. Each player begins the game with two bishops. One starts between the king's knight and the king, the other between the queen's knight and the queen. In algebraic notation the starting squares are c1 and f1 for white's bishops, and c8 and f8 for black's bishops.
The bishops may be differentiated according to which wing they begin on, i.e. the "king's bishop" and "queen's bishop" respectively. However, after a bishop has moved several times, it may be difficult to remember where it came from. Therefore it is more common to refer to the "light-squared" and "dark-squared" bishops, as each always remains on either the white or black squares.
The bishop has no restrictions in distance for each move, but is limited to diagonal movement, forward and backward. Bishops cannot jump over other pieces. As with most pieces, a bishop captures by occupying the square on which an enemy piece sits.
Because the bishop has access to only thirty-two squares of the board, it is rather weaker than the rook to which all sixty-four squares of the board are accessible. Furthermore, a rook on an empty board always attacks fourteen squares, whereas a bishop attacks only seven to thirteen depending on how near it is to the center. A rook is generally worth about two pawns more than a bishop.
Bishops are approximately equal in strength to knights. Bishops gain in relative strength as more and more pieces are traded, and lines open up on which they can operate. When the board is empty, a bishop can operate on both wings simultaneously, whereas a knight takes several moves to hop across. In an open endgame, a pair of bishops is decidedly superior to a bishop and a knight or two knights. A player possessing a pair of bishops has a strategic weapon in the form of a long-term threat to trade down to an advantageous endgame.
On the other hand, in the early going a bishop may be hemmed in by pawns of both players, and thus be inferior to a knight which can hop over obstacles. Furthermore, on a crowded board a knight has many opportunities to fork two enemy pieces. While it is technically possible for a bishop to fork, practical opportunities are rare.
A bishop which has trouble finding a good square for development in the center may be fianchettoed, for example pawn g2-g3 and bishop f1-g2. This forms a strong defense for the castled king on g1 and the bishop can often exert pressure on the long diagonal h1-a8. After a fianchetto, however, the bishop should not be given up lightly, because then the holes around the king can easily prove fatal.
A player with only one bishop should generally place his pawns on squares of the color that the bishop can't move to. This allows the player to control squares of both colors, allows the bishop to move freely among the pawns, and helps fix enemy pawns on squares on which they can be attacked. A bishop which is impeded by friendly pawns is sometimes disparagingly called a "tall pawn", or more simply, a "bad bishop".
In endgames where each side has only one bishop, and the bishops are on squares of opposite colors, a draw becomes more likely. Each side tends to gain control of squares of one color, and a deadlock results. However, if the queens are still on the board, the opposite colored bishops may make the game less drawish, because each side has an extra attacker on squares the other player has trouble defending.
In endgames with same-colored bishops, even a minute advantage may be enough to win.