Kristie
secondary student

Why are perpendicular lines called normal lines?

The best place to go for questions like this is the OXFORD ENGLISH DICTIONARY. The dictionary says that that "norma" is Latin for a carpenter's square. Thus the Romans used a "norma" to draw right angles. Since the time of Newton (in the late 1600s) the word "normal" has been used in English to mean perpendicular. On the other hand, "normal" has so many other meanings that I try to avoid it except in places where it is so common that the meaning is unambiguous (such as the normal to a surface, or when speaking of vectrors as in the terminology "normal basis"). It is more clear (although harder to spell!) if we use either "perpendicular" or "orthogonal" to refer to lines that meet at right angles.