If there is a convention for labeling the vertices of various quadrilaterals ABCD etc. For example ABCD is a parallelogram. E is a point on DC extended, such that D and E are on opposite sides of BC.

If there is a convention for labeling the vertices of various quadrilaterals $ABCD$ etc.
For example $ABCD$ is a parallelogram. $E$ is a point on $DC$ extended, such that $D$ and $E$ are on opposite sides of $BC$.
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Hayden Espinoza
A common convention is that the vertices of a polygon are listed so that adjacent vertices are connected by edges. Thus the edges of quadrilateral $ABCD$ would be $AB$, $BC$, $CD$ and $DA$.