What is a 4th vertex?

Question

im in 8th grade so dont use big words plz!

Answer 1

A vertex is where two straight lines come together.  For instance, a triangle has three sides and three corners.  The three corners are the three vertices (plural of vertex).  A fourth vertex would be a fourth place where two lines come together.  A rectangle, for instance, has four vertices.

I could be more specific if your question were a little more specific.