Sorting by Sides

One way to sort triangles is by how many of their sides are equal. A scalene triangle has no two sides equal. An isosceles triangle has at least two equal sides. An equilateral triangle has all three sides equal. In a figure, equal sides are shown with matching tick marks. Sides with the same number of ticks have the same length.

A scalene, an isosceles and an equilateral triangle, with ticks on the equal sides

An equilateral triangle has three equal sides, so it also has two equal sides. So every equilateral triangle is isosceles as well. The reverse is not true, because an isosceles triangle can have a third side of a different length. For example, a triangle with sides 5, 5 and 8 is isosceles. A triangle with sides 5, 6 and 8 is scalene. A triangle with sides 7, 7 and 7 is equilateral, and also isosceles.