A rook (borrowed from Persian رخ rokh, "chariot") is a piece in the strategy board game of chess. Each player starts with two rooks, one in each of the corners nearest their own side. 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. Like other 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.
Originally, the rook symbolized a chariot. The Persian word rokh means chariot, and the corresponding pieces in Oriental chess games such as xiangqi and shogi have names meaning chariot. However, in the West, the rook is almost universally represented as a fortified tower. One possible explanation is that when the game was imported to Italy, the Persian rokh became the Italian word rocca, meaning fortress. Another possible explanation is that rooks represent siege towers. Rooks are usually made to look like small castles, and as a result, a rook is sometimes called a "castle". This usage was common in the past ("The Rook, or Castle, is next in power to the Queen" -- Howard Staunton, Blue Book of Chess, 1847) but no longer (unless very informally); the verb 'to castle' is however still the only correct term used to refer to the double move of king and rook.
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 generally considered to be 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 (see Chess piece point value).
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 on 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. These rooks are sometimes colloquially referred to as "pigs on the seventh", because they often threaten to "eat" the opponent's pieces or pawns.
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.
In heraldry, chess rooks are often used as charges. Unlike a real chess rook, they are conventionally shown with two outward-curving horns. This is because they would otherwise appear to be castle towers, since there is no proportion on a coat of arms. This charge is always blazoned "chess rook" so as not to be confused with the bird of that name; it is also not to be confused with the zule, a similar-looking object with two outward-curving horns at both top and bottom.
In Canadian heraldry, the chess rook is the brisure of the fifth daughter.