Why is a right angle called a right angle? Why not a left angle or straight angle or whatever?
“Right” in the sense of “correct”. Note that US carpenters use the terms “square”, “plumb” and “true” for correctly setup angles (such as a door frame). I assume that “right” and “true” are synonyms in this context.
From the The American Heritage® Dictionary of the English Language: Fourth Edition. 2000. entry for right:
Ever hear of somethin righting itself? (Example : a ship listing to one side rights itself when the ballast is shifted.) A reflex SWAG, the two things might be related: right = vertical?