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 Description  
 Trapezoid:  A quadrilateral that has only one pair of parallel sides.

The parallel sides of a trapezoid are called bases. The angles that share a base as a common side are called a pair of base angles.

If the sides of a trapezoid that are not parallel are exactly the same length, the trapezoid is said to be isosceles. Where have you seen that term before? Compare the base angles of an isoceles trapezoid. Now look at the diagonals of an isoceles trapezoid. Compare their lengths.

Trapezoidal shapes are often used in building arches, where each block or stone supports, and is supported by, the pieces above and below it. Another use is in binoculars in which the magnifying lenses invert the image. Trapezoidal prisms are used to turn the inverted images right side up again.