What is a right angle?
A right angle is an angle of exactly 90 degrees, a quarter of a full turn. It is formed when two lines meet perpendicularly, like the corner of a square or a book.
Right angles are everywhere: in the corners of rooms, windows, doors, sheets of paper and most buildings. In diagrams, a right angle is marked with a small square in the corner rather than a curved arc. Lines that meet at a right angle are called perpendicular.
Angles smaller than 90 degrees are called acute angles, and angles between 90 and 180 degrees are called obtuse angles. An angle of exactly 180 degrees is a straight angle. A triangle with one right angle is called a right triangle, and its longest side, opposite the right angle, is the hypotenuse.
Making right angles
Builders and carpenters use tools called set squares and try squares to check right angles. Ancient Egyptians are thought to have used ropes with 12 evenly spaced knots, forming a triangle with sides of 3, 4 and 5 units, which automatically creates a right angle.
Did You Know?
The word right in right angle comes from the Latin rectus, meaning upright or straight, not from the direction right.