The line between two nonadjacent points in a square. According to the query of Mathematics Education Network, the diagonal of a square is the connecting line of two non-adjacent points of the square. Diagonal lines of a square have the property that the two diagonal lines of a square are equal in length. Two diagonals of a square intersect at a point, and the two diagonals are equally divided. The two diagonal lines of a square are perpendicular to each other.