In an isosceles trapezoid, the base angles are equal, and so are the other pair of opposite sides AD and BC. Other authors define a trapezoid as a quadrilateral with at least one pair of parallel sides, making a parallelogram a special type of trapezoid. Some authors define a trapezoid as a quadrilateral having exactly one pair of parallel sides, thereby excluding parallelograms. At issue is whether parallelograms, which have two pairs of parallel sides, should be counted as trapezoids. There is also some disagreement on the allowed number of parallel sides in a trapezoid. Readers in the UK should read trapezium for each use of trapezoid in the following paragraphs. This article uses the term trapezoid in the sense that is current in the USA and some other English-speaking countries. This sense is the one that is standard in the U.S., but in practice quadrilateral is used rather than trapezium. The sense of a trapezium as an irregular quadrilateral having no sides parallel was the usual sense in England from c1800 to c1875, but is now rare. A trapezium as any quadrilateral more general than a parallelogram is the sense of the term in Euclid. This was the specific sense in England in 17th and 18th centuries, and again the prevalent one in recent use. A trapezium in Proclus' sense is a quadrilateral having one pair of its opposite sides parallel. Īccording to the Oxford English Dictionary, the trapezoid as a figure with no sides parallel is the sense for which Proclus introduced the term it is retained in the French "trapézoïde", German "trapezoïd", and in other languages. The term trapezoid has been defined as a quadrilateral without any parallel sides in Britain and elsewhere, but this does not reflect current usage (the Oxford English Dictionary says “Often called by English writers in the 19th century”). In North America, the term trapezium is used to refer to a quadrilateral with no parallel sides.
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