A square is a shape with four equal [side]]s and four corners that are all right angles (90 degrees). The diagonals of a square also cross at right angles. The angle between any diagonal and a side of a square is 45 degrees. A square has rotational symmetry of four. It has four lines of regular symmetry.
A square is also the 2-dimensional analogue of a cube.
If the length of side a is known:
|Length of diagonal|
If the length of diagonal d is known:
|Length of side|