Possible Duplicates:
What is the difference between 20$ and $20?
Why is the unit of measure placed before the value for currencies? Are there other measures where the unit precedes value?

We express fifty dollars, not as "50$", but rather as "$50". Other units are expressed with a trailing symbol (20¢, 10m, 5kWh, 6oz). Why are dollars treated specially; what's the history behind this?