<Li> 6 hours (astronomical hour angle) </Li> <P> Throughout history carpenters and masons have known a quick way to confirm if an angle is a true "right angle ." It is based on the most widely known Pythagorean triple (3, 4, 5) and so called the "Rule of 3 - 4 - 5 ." From the angle in question, running a straight line along one side exactly three units in length, and along the second side exactly four units in length, will create a hypotenuse (the longer line opposite the right angle that connects the two measured endpoints) of exactly 5 units in length . This measurement can be made quickly and without technical instruments . The geometric law behind the measurement is the Pythagorean theorem ("The square of the hypotenuse of a right triangle is equal to the sum of the squares of the two adjacent sides"). </P> <P> Thales' theorem states that an angle inscribed in a semicircle (with a vertex on the semicircle and its defining rays going through the endpoints of the semicircle) is a right angle . </P> <P> Two application examples in which the right angle and the Thales' theorem are included (see animations). </P>

One and a half times a right angle