A right triangle is a triangle with one angle that measures exactly 90 degrees, like the corner of a square or rectangle. That 90-degree angle is called the right angle, and it's the defining feature that makes this triangle special among all triangles.

Picture folding a square piece of paper diagonally: you've just created two right triangles. The two sides that form the right angle are called legs, while the longest side, opposite the right angle, is called the hypotenuse.

Right triangles appear everywhere in the world around you. Carpenters use them to make sure walls stand perfectly straight. The supports under a staircase often form right triangles. Even the ancient Egyptians used right triangles to build the pyramids, ensuring their corners were perfectly square.

The Greek mathematician Pythagoras discovered something remarkable about right triangles: if you square the length of each leg and add those numbers together, the sum always equals the square of the hypotenuse. This relationship, called the Pythagorean theorem, has been one of mathematics' most useful tools for over 2,500 years. Engineers, architects, and navigators still use it to calculate distances and plan structures. What looks like a simple three-sided shape turns out to be one of geometry's most powerful and practical tools.