A right triangle, also known as a right-angled triangle, always has one angle at 90 degrees. The side directly opposite this angle is known as the hypotenuse, which is the longest side of a triangle.
If the lengths of two sides of a triangle are known, it is possible to calculate the length of the third side using Pythagoras' theorem. This theorem states that the sum of the square of the hypotenuse is equal to the sum of the squares on the other two sides. This theorem only applies to right triangles.
When a right triangle has sides with lengths that are each an integer, it's called a Pythagorean triangle.