From Chesspedia, the Free Chess Encyclopedia.
A bishop (♗♝) is a piece in the board game of chess. Each player begins the game with two bishops, one light-squared and one dark-squared. 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 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.
The bishops may be differentiated according to which wing they begin on, i.e. the king's bishop and queen's bishop. As a consequence of its diagonal movement, each bishop always remains on either the white or black squares, and so it is also common to refer to them as light-squared or dark-squared bishops.
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 towards the endgame 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.
|After White's 23rd move of Krasenkow vs. Zvjaginsev, FIDE World Chess Championship 2004, a thicket of black pawns hems in Black's bishop, so Black is effectively playing with one piece fewer than White. Although the black pawns also obstruct the white bishop, it has many more attacking possibilities, and thus is a good bishop vis-a-vis Black's bad bishop. Black resigned after another ten moves.|
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. A fianchettoed bishop should not be given up lightly, because then the holes in the pawn formation around the king can easily prove disastrous.
A player with only one bishop should generally place their pawns on squares of the color that the bishop cannot 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". However, a "bad" bishop may not be really bad, if it is outside its own pawns' pawn chains. A bad bishop can be made good by getting it outside the pawn chain.
An endgame in which each player has only one bishop, one controlling the dark squares and the other the light, will often result in a draw even if one player has a pawn or two more than the other. The players tend to gain control of squares of opposite colors, and a deadlock results. In endgames with same-colored bishops, however, even a minute advantage may be enough to win.
The one unknown which remains is the design of the slit in the upper part of the piece, specifically, why it exists.