The dollar sign is displayed with a single & at other times double vertical line, why & do they have names?

Answer 1

I found a lot of interesting theories on the origin of the symbol here:

http://www.ex.ac.uk/~RDavies/arian/dollar.html

The "pesos" one seems to be the most popular.

But as of yet am unable to explain why the $ sometimes is single and sometimes is double.

Answer 2

The original sybol was on the Spanish silver coin that could be split into 8 pieces or "pesos de ochas" the reverse had the Serpent entwined around two pilars and became the sybol of the Dollar throughout the new world.

Answer 3

the single stroke dollar sign was created between the britsh americans and mexico to resemble the PS" (for 'peso' or 'piastre') which is the spanish dollar. Later it was changed by the U.S. to resemble to U and S (put them on top of one another) and that is what we use to reference US currency!