A diagonal in chess is a line of same-colored squares running at an angle across the board, along which bishops and queens move.
The board contains 26 diagonals in total — 13 on dark squares and 13 on light squares. The longest ones, known as the main diagonals, run from a1 to h8 (dark squares) and from a8 to h1 (light squares), each spanning eight squares. The shortest diagonals consist of a single corner square and have no practical relevance.
Understanding diagonals is essential for grasping how long-range pieces operate. A bishop placed on a main diagonal can control up to seven squares at once, applying pressure across the entire board. This is why the a1-h8 and a8-h1 diagonals are so highly contested — controlling them allows a piece to threaten squares on the opposite side of the board.
In practice, always identify which diagonals are open (unobstructed by pawns or pieces) in your position. If an opponent’s bishop occupies a long diagonal aimed at your king, look to close it by advancing a pawn onto a square of that diagonal or by interposing a piece.