A square is the regular 4-gon, that is, a quadrilateral whose 4 angles and 4 sides are respectively equal. Each angle of a square must be a right angle. This implies a square is a parallelogram that is both a rhombus and a rectangle at the same time.

Notice, however, that if a quadrilateral has its 4 sides equal, we cannot generally say it is a square, since it might be a rhombus as well.

If is the length of a side, the diagonals of a square (which are equal since it's a rectangle too) have length .