Explanation:
Quadrilateral
In Euclidean plane
geometry, a quadrilateral is a polygon with four sides (or 'edges') and four vertices or corners.
Sometimes, the term quadrangle is
used, by analogy with triangle,
and sometimes tetragon for consistency with pentagon (5sided), hexagon (6sided) and so on. The word quadrilateral is made of the words quad (meaning "four") and lateral (meaning "of sides").
Quadrilaterals are simple (not selfintersecting) or complex (selfintersecting), also called
crossed. Simple quadrilaterals are either convex or concave.
The interior
angles of a simple quadrilateral
add up to 360 degrees of arc. (Our solved example in
mathguru.com uses this concept)
This is a special case of the ngon
interior angle sum formula (n 
2)180°. In a crossed quadrilateral, the interior angles on either side of the
crossing add up to 720°.
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Quadrilateral

Six different types of quadrilaterals

Edges and vertices

4

Schläfli symbol

{4} (for square)

Internal angle (degrees)

90° (for square)

http://en.wikipedia.org/wiki/Quadrilateral
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above explanation is copied from Wikipedia, the free encyclopedia and is
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License.