In a right triangle, the two legs are the sides that form the right angle.
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Historical note
In early treatises on geometry, the term cathetus generally referred to the height of a figure.
Towards the 10th-11th century, Pope Sylvester II (Gerbert d’Aurillac) wrote, in his treatise on geometry :
“A right trapezoid is simply referred to as “a trapezoid.” It has a base, a coraustus, or small base, and a cathetus.”