What is Diagonal?

A diagonal is a line of squares of the same colour running obliquely across the board. Bishops and queens operate along diagonals. There are 26 diagonals in total—13 light-square diagonals and 13 dark-square diagonals. The two longest diagonals (a1–h8 and a8–h1) are called the long diagonals, each containing 8 squares.