Definition:Triangle (Geometry)/Nomenclature

Definition

The vertices of a triangle are conventionally labeled $A, B, C$ (or with other uppercase letters), and the sides with lowercase letters corresponding to the opposite vertex, as above.

In order to emphasize that a particular vertex being referred to is in fact a vertex, the symbol $\angle$ is often placed by the letter corresponding to that vertex.

Thus, for example:

$\angle A$ is adjacent to sides $b$ and $c$

Side $a$ is adjacent to $\angle B$ and $\angle C$

$\angle A$ is opposite side $a$

Side $a$ is opposite $\angle A$.