**When the lengths of all sides of a triangle are added, the result is called the perimeter of the triangle.** In general, a perimeter is the distance of the curve that borders a lamina or a two-dimensional closed planar surface. Half of a triangle's perimeter is called the semiperimeter.

The sides of a triangle can be used to calculate other properties of the triangle. For instance, the area of a triangle can be computed using Heron's formula, which requires all sides and the semiperimeter as given. The lengths of the sides, in conjunction with the measurements of the angles, are also important factors in triangle geometry, inequalities and trigonometry.