trapezoid is a quadrilateral with exactly one pair of parallel sides. Therefore, a trapezoid is not a parallelogram. The parallel sides are called bases and the nonparallel sides are called legs.


Since there are two bases in a trapezoid, then there are two pair of base angles.  In the trapezoid above, one pair of base angles is  angle A and B, the other pair is angle C and D. 


A midsegment of a trapezoid is a segment whose endpoints are the midpoints of the legs and is parallel to the bases.  The length of the midsegment is half the sum of the length of the bases or the average of the length of the bases.