Yet another Yank/Brit difference I never knew about. It suddenly occurred to me to wonder why a cross-bar suspended by ropes for acrobatic purposes was called a trapeze. I went to the OED, which said “Prob. orig. applied to a kind in which the ropes formed a trapezium (in sense 1b) with the roof and cross-bar.” So I went to trapezium and found:
1. Geom. a. Any four-sided plane rectilineal figure that is not a parallelogram; any irregular quadrilateral. (The Euclidean sense.)
b. spec. A quadrilateral having only one pair of its opposite sides parallel. (The specific sense to which the term was restricted by Proclus.)
The specific sense in Eng. in 17th and 18th c., and again the prevalent one in recent use.
c. An irregular quadrilateral having neither pair of opposite sides parallel. (The usual sense in England from c1800 to c1875. Now rare. This sense is the one that is standard in the U.S., but in practice quadrilateral is used rather than trapezium.)
This is the trapezoid (τραπεζοειδές) of Proclus: see TRAPEZOID A. 1a.
What a mess! The etymology, after explaining that the Greek etymon trapezion is a diminutive of the word for ‘table,’ trapeza, has a long small-type paragraph that describes the shift in meaning from Euclid’s (a above) to Proclus’s (b) and then adds:
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