I learned about left angles from a third grader. While identifying angles – acute, obtuse, and right – he was stumped by the picture of a right triangle. He said, "It isn't any of them." I asked him what it was and he replied, "It's a left angle." I looked back at all of the right triangles he had encountered in the lesson; their right angles were all on the right side. When faced with a right angle on the opposite side, he used his prior knowledge of opposites and decided it was a left angle. Because he could explain his thinking, I was able to clear up his confusion.
As I wrote this entry, I realized I had no idea why a 90 degree angle is called a right angle, so I googled it. According to Dr. Michel Smith, professor and chair of the Mathematics Department in Auburn University's College of Sciences and Mathematics, "The word "right" is used for this angle because the word has the meaning of "true" or "correct." When a carpenter is building a house and he positions a wall, he wants that wall to be correctly placed; to be "right" or "true." So when the wall is vertical then it is correctly placed or "true." A correctly placed "true" wall is perfectly vertical and so it makes a "right" angle with the ground. This is what a right angle or a 90 degree angle is."
I learned something new today! I think I will follow the advice in my prior post – teachers should be less helpful – by asking my students to find the origin of the term right angle for themselves.