A truss is a structure made up of triangles. There are three main reasons that triangles are used to form trusses: their unique geometric properties, their method of transferring loads and their spatial openness.

### Rigidity

A triangle has three sides and three angles, and each angle is held solidly in place by the side opposite it. This means that a triangle's angles are fixed, and that if pressure is placed anywhere on a triangle, its angles, unlike those of other shapes, will not change.

### Forces

When a force is applied to a triangle, the resulting pressure is directed sideways rather than down. This means that sides of the triangle are in either compression or tension, and that there is thus no bending movement.

### Openness

Since the centre of a triangle does not contribute to its geometric rigidity or structural integrity, the centre of a triangle can remain open. As one of the goals in erecting a truss is minimising its weight, triangles are an appropriate shape to use.