Triangle that has at least two congruent sides.

### Properties

- An isosceles triangle has at least one line of symmetry. If it has more than one, then it is an equilateral triangle.
- A non-equilateral isosceles triangle has two congruent angles, which are opposite its congruent sides.If its three angles are congruent, then it is equilateral.

### Notation

In the representation of an isosceles triangle, the congruent sides are often marked with hash marks to make them easier to identify and discuss.