The most fundamental property of any quadrilateral is that the sum of its four interior angles is always exactly [b]360°.[br]Drag the vertices of this quadrilateral along the circumscribed circle and check that the four angles always add up to 360 degrees:[/b]

[list][*][b]Opposite angles[/b] are equal [/*][*][b]Consecutive angles[/b] are supplementary (they add up to 180°)[br][/*][/list]Draw a parallelogram and measure its angles to verify this property.

All four interior angles are [b]right angles[/b] (exactly 90° each).[br]Draw a rectangle and measure its angles to verify this property.