A square has four right angles (90°). Each diagonal splits a corner into two angles of 45°. Note also that the diagonals cut each other in half at right angles. A rectangle has four right angles.

For any quadrilateral, the angles of the four corners add up to \(360^\circ\). The diagonals meet at right angles so we know we have an angle of \(90^\circ\). The three angles in a triangle add up ...