Definition: The point at which two lines or line segments meet each other is called Vertex
Plural of Vertex is referred to as Vertices
In the following figure:
There are two lines  Line 1 and Line 2.
When both these lines meet each other at Points O, we get the following figure:
Let study following figure also and find names of Vertices:
Figure 1:
In the above figure, A, B, C & D are the Vertices of given figure.
Figure 2
In the above figure, A, B, C, D, E & F are the Vertices of given figure.
Figure 3:
In the above figure, P, Q, R & S are the Vertices of given figure.
