An isosceles triangle is a triangle with two sides that are exactly the same length. The word comes from Greek roots meaning “equal legs,” which makes sense when you picture it: the two matching sides look like legs propped up at an angle, meeting at a point at the top.

If you draw an isosceles triangle, you'll notice something interesting about the angles too. The two angles at the base (where the equal sides meet the bottom side) are always the same size. In a triangle with two equal sides, the angles opposite those sides are equal.

Mathematicians and engineers care about isosceles triangles because they show up everywhere in the real world: in roof trusses, bridge supports, sailboat sails, and even in the way light reflects off mirrors. When you fold a piece of paper in half and cut it at an angle, you often create an isosceles triangle without even trying.

Once you know the word, you'll start noticing isosceles triangles everywhere: in pizza slices and architecture. It's one of the three main types of triangles you'll study in geometry, along with equilateral (all three sides equal) and scalene (no sides equal).