a river is used in Korean janggi. Some chess variants use more than a single board per match. Bughouse chess, for example, involves four players playing two simultaneous matches on separate boards. Alice Chess is a popular variant which is usually played on two boards to facilitate the movement of pieces between the boards. Three-dimensional boards are often represented by multiple two-dimensional boards. Variants may use anywhere from two to eight boards. For example, Raumschach utilises five boards of twenty-five squares each, totaling 125 squares. Another noteworthy variant, Star Trek Chess, utilises a board of sixty-four squares divided into seven levels. Star Trek Chess uses a board with movable parts divided into seven levels. In the initial position, each player occupies two of the movable four-square attack boards. The white pieces start in the lower level, using attack boards connected to this level and the first two rows of the board, while the black pieces start at the top, using the attack boards and first two rows of the third level. Other representations The game of chess has been represented in the arts since its creation. Chess sets usually had considerable artistic value; they were made of noble materials, such as ebony and ivory, and in large sizes. Many of the pieces in these sets were offered to churches as relics. The book Liber miraculorum sancte Fidis tells a story in which a nobleman, after miraculously escaping from prison, is forced to carry a chessboard until a sanctuary as gesture of gratitude. However, more frequently, there are stories in which the chessboard is used as a weapon. The French tale of Ogier the Dane reports how the son of Charlemagne brutally kills one of Ogier's sons with a chessboard after losing a match, although there is no evidence confirming the veracity of the story. In 1250, a sermon called Quaedam moralitas de scaccario per Innocentium papum (The Innocent Morality) showed the world as being represented by a chessboard. The white and black squares represented the two conditions of life and death or praise and censure; over these, the pieces, representing humanity, would confront each other in the adversities of the game, which symbolised life. Due to its simple geometry, the chessboard is often used in mathematical puzzles or problems unrelated to chess, such as the wheat and chessboard problem and the mutilated chessboard problem. The term infinite chessboard is sometimes