A **triangle** is a three‐sided figure with three angles in its interior. The symbol for triangle is Δ. A triangle is named by the three letters at its vertices (the plural of vertex), a fancy name for corners. This is Δ *ABC* (Figure 1).

**Figure 1 **A triangle.

**
**

As you can imagine, the measuring of triangles and more complex figures became important long ago because of their role in surveying. Modern science has continued to find more and more practical applications requiring knowledge of triangles.

Note that any closed figure in the plane with three or more sides can be subdivided into triangles (see Figure ). Consequently, what you learn about triangles can also be useful in studying more complex figures.