Ranks and files are not the only highways of the chessboard. There are also the diagonals, which are straight lines made up of individual squares that border at the corners, rather than at the sides. They appear to stretch out at an angle from the players’ perspective.

The following three main things distinguish a diagonal from a rank or file.

• Diagonals border at the corners rather than at the sides.

• Diagonals come in a variety of sizes, whereas ranks and files always contain eight squares each.

• Diagonals consist of squares of one color only, whereas ranks and files always contain an equal mixture of dark and light squares.

Diagonals do not have easy-to-remember, simple names like ranks and files do. But they are sometimes named for the first and last square on the diagonal: the long dark diagonal can be called the a1-h8 diagonal, while the smallest light-square diagonals can be called either the h7-g8 diagonal or the a2-b1 diagonal.

Border

Diagonals border at the corners rather than at the sides of the squares that make them up. This brings up an interesting optical illusion. Look at a chessboard. Consider the a-file and a1-h8 diagonal. Which is longer?

If you answered the diagonal, you were right in a strictly geometrical sense, but wrong in a chess sense. Each row contains eight squares, and that means that they are the same size for the purpose of a chess game. By the same token, it might look like the b1-h7 diagonal is longer than the b-file. But actually it is the file that is longer! The b-file, like all files, contains eight squares, whereas the b1-h7 diagonal consists of only seven squares.

Identifying diagonals

Size

Thus you can see a very important property of diagonals: They are not even close to being equal. Diagonals are made up of anywhere from two to eight squares. There are four diagonals containing two, three, four, five, six, and seven squares (two dark and two light), while there are two long diagonals, which each contain eight squares (one dark and one light).

The most important property of diagonals is that they are all made up of squares of one color. Diagonals are checkers’ highways! There can never be a dark square on a light-square diagonal. Thus diagonals are limited-access highways compared to ranks and files.