For a long time, historians and mathematicians believed it was a stylized version of an "S" superimposed on a "U". However, in the 1920s the Department of Treasury concluded that "$" is actually a lineal descendant of the Spanish abbreviation "ps" for "pesos." This idea -- of an "S" being written over a "P" eventually replaced the "US" theory of the symbol's origin. While neither grouping of letters seem to resemble "$" more than the other, the fact that Spanish and Mexican coins were the main currency in many parts of the US in the 18th and 19th centuries seems to lend credence to the latter theory. So next time you wake up fuzzyheaded and dizzy after a night out on the town and discover just how little $ you have left, take comfort in knowing that you can delight and impress your equally broke friends with this bit of money trivia.