Base 10 notation Why are the ten digits in base 10 noted $0, 1, 2 ,3 ,4 ,5 ,6 ,7 ,8 ,9$ ? I heard it has to do with angles, but since I can't see how $9$ has $9$ angles, this may not be the historical reason ?
 A: It has nothing to do with angles. The digits we have now are alterations of the Arabic versions, which in turn are modified versions of the original Hindi symbols in a script called Devanagari:

(I nabbed this diagram from the Wikipedia article on digit symbols.)  
Florian Cajori History of Mathematical Notations Vol. 1 (Open Court, 1928) has an extensive explanation of the development of digit symbols on pages 45–70. According to Cajori, the Hindi system dates to sometime between 1100 and 1300 years ago; the symbols were adopted in Europe only about 800 years ago, although their earliest use in Europe dates as far back as the year 976. This table of early numeral forms is from Cajori p. 50:

If you look at the letter shapes we use now, you can see similarly drastic alterations over the course of two thousand years, as the letters were adopted by the Phoenicians from Egyptian hieroglyphic, then by the Greeks from the Phoenicians, then by the Romans from the Greeks, and then by medieval Europeans from the Roman forms. 
A: You might be referring to this info graphic? Of course, this is creatively made up.

A: Only the origins of $0, 1, 2, 3$, and $4$ are clear. In their initial form, the numeral $1$ is indicated by one vertical J-like stroke, $2$ by two such joint strokes, $3$ by three of them, and $4$ is a four-armed cross. If the original $2$ and $3$ are rotated at a $90$-degree angle, they look very close to their current form. If one of the arms of the initial $4$ is rounded a bit, it looks very similar to its modern shape. $0$ comes from an empty square acting like a placeholder for a non-existing quantity in the Hindu positional notation.
