Right angle

A right angle measures exactly 90 degrees, or π/2 radians. It is formed by perpendicular lines or rays and marks a quarter-turn.

Angle: A geometric figure formed by two rays sharing an endpoint, or by two intersecting lines. A right angle is a specific kind of angle, distinguished by its measure.
Pythagorean theorem: A theorem stating that the square of a right triangle’s hypotenuse equals the sum of the squares of its legs. Its converse can establish that a triangle contains a right angle.
Cartesian coordinate system: A system that locates points using signed distances from perpendicular axes. Its horizontal and vertical axes meet at the origin at right angles.
Acute angle: An angle measuring more than 0 degrees and less than 90 degrees. It is smaller than a right angle.
Degree (angle): A unit of angular measure equal to one three-hundred-sixtieth of a full turn. Ninety degrees is the standard measurement used to define a right angle.
Thales' theorem: A theorem stating that an angle inscribed in a semicircle is a right angle. A diameter and a point on its circle construct a right angle.
Rectangle: A quadrilateral with four right angles. Every corner of a rectangle is a right angle.
Obtuse angle: An angle measuring more than 90 degrees and less than 180 degrees. It is larger than a right angle but smaller than a straight angle.
Radian: A unit of angular measure equal to the angle subtended by an arc as long as its circle’s radius. The same quarter-turn measures π/2 radians.
Carpenter's square: A two-armed measuring tool with arms fixed at a right angle. Its fixed corner provides a physical reference for checking right angles.

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