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Language leads them astray.

Having drawn one right angle on the blackboard… then a second back-to-back with it – to show that two right angles together make a straight line. One student asks… “isn’t that one a left angle”?

Apparently, “The term is a calque of Latin angulus rectus; here rectus means “upright”, referring to the vertical perpendicular to a horizontal base line.”

Seems that I should start using Latin in class, would certainly get an occasional laugh.