# Convention for labeling vertices of quadrilaterals.

I am wondering 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.

Normally I put A in the top left then B in the top Right then C in the bottom left then D in the bottom right but it seems sometimes this results in incorrect construction.

Thanks!

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Absent any explicit indications I would assume that the vertices of a polygon are labeled in consecutive (i.e. clockwise) order rather than in reading order. – user7530 Jan 1 '13 at 22:33

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 $DA$. So if it's a parallelogram, it might look like

A --- B          A --- D
\     \     or   \     \
D --- C          B --- C

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Agreed, Robert. Usually I label counterclockwise because that is the positive orientation in the plane. – ncmathsadist Jan 1 '13 at 22:36
+1 nice pictures! – amWhy Jan 1 '13 at 22:46

I believe counterclockwise is the normal labeling convention - avoiding uncertainty as to what the polygon would look like.

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