= Quadrilateral
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A <quadrilateral> is a <polygon> with four sides and four <vertices of a polygon>. For a simple <quadrilateral>, its interior <angles> sum to $2\pi$ by dividing it along an interior <diagonal of a polygon> into two <triangles>. <Rectangles>, <squares> and <parallelograms> are special cases. The qualifier <convex quadrilateral> excludes a reflex interior <angle>; a self-crossing four-segment chain requires a different interior convention. Four side lengths alone generally leave freedom to vary the <angles> and <area>.
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