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

from Wikipedia

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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    A$n$other common notation is a square (or rather half-square). No dot. Its provenance is obvious.2011-10-20

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