Quadrilateral
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In geometry, a quadrilateral is a polygon with four sides and four vertices. Quadrilaterals are further classified as follows:
- Trapezium: two of the opposite sides are parallel.
- Isosceles trapezium: two of the opposite sides are parallel, the two angles incident with each side are equal.
- Parallelogram: opposite sides have equal length, opposite sides are parallel, opposite angles are equal.
- Kite: two adjacent sides have equal length, the other two sides have equal length, opposite angles are equal.
- Rhombus: four sides have equal length, opposite sides are parallel, opposite angles are equal.
- Rectangle: opposite sides have equal length, opposite sides are parallel, each angle is a right angle.
- Square (regular quadrilateral): four sides have equal length, opposite sides are parallel, each angle is a right angle.
http://www.harcourtschool.com/glossary/math/images/quadrilateral.gif
Better illustrations needed.
Also the name of a group of fortresses in Italy.