# History of mathematical symbols, especially the symbol for right angle

Yesterday a child asked me, why (historically) a right angle is denoted by an arc and a dot like in this picture:

I dont't know it, but I am interested in it too, so I post this question to this site.

This lead me to ask also the more general question if there are any good references which deal with the history of mathematical notation and symbols in general.

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The canonical answer for the last question: Jeff Smith's page Earliest Uses of Various Mathematical Symbols. The geometry section doesn't seem to answer your first question, though. –  t.b. Oct 20 '11 at 16:58
The second canonical answer is Floran Cajori's A History Of Mathematical Notations Vol I (1928). Unfortunately, section 363 on special angles does not answer your question, but a few related ones. –  t.b. Oct 20 '11 at 17:11
This is the first time I've seen right angles indicated that way. The convention I'm accustomed to indicates a right angle with a tiny square like the image here on the left... –  Guess who it is. Oct 20 '11 at 17:11
@'J.M.' According to the german wikipedia (de.wikipedia.org/wiki/Rechter_Winkel) this is a notation which is common in germany and some other european countries. –  student Oct 20 '11 at 17:27
Another common notation is a square (or rather half-square). No dot. Its provenance is obvious. –  Yuval Filmus Oct 20 '11 at 19:11