1) Bracket shape 2) Ell feature 3) Part of a square 4) Specialty of Pythagoras 5) Square feature 6) Square part 
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles. As a rotation, a right angle corresponds to a quarter turn (that is, a quarter of a full circle)....
(from the article `geometry`) Ancient builders and surveyors needed to be able to construct right angles in the field on demand. The method employed by the Egyptians earned them ...
[n] - the 90 degree angle between two perpendicular lines
An angle of 90°.
The angle formed when two straight lines intersect and the 4 angles at theircrossing are all equal.right angle = p/2 radians = 90°
An angle formed by perpendicular lines, line segments, or rays and with a measure of 90° 
noun the 90 degree angle between two perpendicular lines
An angle equal to 90°.