Three Sides, Three Corners

Part of the Triangles section of Coddy's Geometry journey. Lesson 1 of 71.

Take three points that are not on one straight line and join them with three segments. The shape you get is a triangle. The three points are its vertices, and the three segments are its sides. A triangle is named by its vertices, so this one is triangle ABC. Each side is named by its two endpoints: AB, BC and CA.

$Triangle ABC with its sides labelled AB, BC and CA$

Triangle ABC with its three vertices and three sides

Each vertex has one side that it does not touch. That side is called the side opposite the vertex. Vertex A is opposite side BC, vertex B is opposite side CA, and vertex C is opposite side AB. Many rules about triangles use the side opposite a vertex, so make sure you can find it. The last section was about the angles at the vertices. This section is about the sides: their lengths and how they fit together.