How many lines?

Some are saying it was always written with one vertical line, some with two and some with both.

The origin of the vertical line is an interesting story itself. Originally, the US used Spanish Peso, which was written P with a supercripted "S". Writing PS so many times caused them to merge into the S with a line though.

Sometimes, however, it appears with 2 vertical lines, but there's no doubt the single line one seems dominant - just take a look at your keyboard, for example ;-)

There is a variation of the origin story which is simpler. It just says it originated by writing U directly over S, cutting out the bottom of the U. This would result in two vertical lines. Even more curious is the UK pound symbol is sometimes seen with two horizontal lines, and sometimes with one...

Money Bags

The notion the two lines came from compacting USA to the point all the letters overlapped could have come from the way money bags were originally used by the US mint.

Another suggestion is the symbol is from the digit 8 which has been cut in half, as in Spanish "pieces of 8". The version with two lines is often called a cifrao, and was used as the symbol for the currency of Portugal before it adopted the Euro.