An isosceles triangle is a triangle with at least two sides that are exactly the same length. The word comes from ancient Greek: isos means equal and skelos means leg, so it literally means “equal legs.”

If you cut a slice of pizza from the center to the edge and then cut it in half lengthwise, you'd probably get an isosceles triangle. The two sides running from the tip to the crust would be equal, while the bottom crust edge would be different. The two equal sides are called legs, and the third side is called the base.

Because two sides are equal, the two angles at the base are also equal to each other. This is a fundamental property that mathematicians have known about for thousands of years. If you measure those base angles with a protractor, they'll match.

Isosceles triangles show up constantly in architecture, engineering, and design because their symmetry makes structures stable and pleasing to look at. The roof of many houses forms an isosceles triangle when viewed from the side. Many bridges use isosceles triangles in their support structures because the equal sides distribute weight evenly.