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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
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1 Answer

up vote 2 down vote accepted

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
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