Kings and the opposition

When two kings face each other with one square between them, the player who does not have to move holds the opposition and can force the other to give way. Count the squares between the kings: with one empty square between them, the side to move must step aside.

Opposition is parity in disguise. Whoever can pass the turn keeps control, and since you cannot pass on a chessboard, the spacing decides who is squeezed. Direct opposition is the kings on one line with a single gap; the same counting extends to diagonal and distant opposition.