A rook is a piece in the strategy board game of chess. Each player starts with two rooks, one in each of the corners nearest him. In algebraic notation, white's rooks start on a1 and h1, while black's rooks start on a8 and h8
The rook moves horizontally or vertically, forward or back, through any number of unoccupied squares, as shown in the diagram at left. Like most pieces, it captures by occupying the square on which an enemy piece stands. The rook also participates, along with the king, in a special move called castling.
In general, rooks are stronger than bishops or knights and are consequently considered about two pawns greater in value. Winning a rook for a bishop or knight is referred to as winning the exchange. Two rooks are worth slightly more than a queen. Rooks and queens are called heavy pieces or major pieces, as opposed to bishops and knights, which are called minor pieces.
In the opening, the rooks are undefended by other pieces, so it is usually desirable to unite one's rooks on the first rank by castling and clearing all pieces except the king and rooks from the first rank. In that position the rooks protect each other, and can easily move to threaten the most favorable files.
A common goal with a rook is to place it on the first rank of an "open" file, i.e. one unobstructed by pawns of either player, or a "half-open" file, i.e. one unobstructed by friendly pawns. From this position, the rook is relatively unexposed to risk but can control every square in the file. If one file is particularly important, a player may advance one rook on it, and move the other behind, doubling the rooks.
A rook on the seventh rank (the opponent's second rank) is usually very powerful, as it threatens the opponent's unadvanced pawns and hems in the enemy king. Two rooks on the seventh rank are often enough to force victory, or at least a draw by perpetual check.
Rooks are most powerful towards the end of a game, where they can move unobstructed by pawns and control large numbers of squares. They are somewhat clumsy at restraining enemy pawns from advancing towards promotion, unless they can occupy the file behind the advancing pawn. By the same token, a rook best supports a friendly pawn towards promotion from behind it in the same file.