If you’re wondering where the dollar sign ($) came from, you’re in good company. No one really knows for sure, and several theories have emerged over the years. The most widely accepted explanation, according to the Bureau of Engraving and Printing, goes back to the Spanish peso, which was accepted as the basic unit of value in colonial America during the late 1700s.

Handwritten manuscripts dating to that time show that the peso—formally “peso de ocho reales” or “piece of eight” in America—was abbreviated PS. It’s believed that as time went on, the abbreviation was often written so that the S was on top of the P, producing an approximation of the $ symbol. The $ first appeared in print after 1800 and was widely used by the time the first U.S. paper dollar was issued in 1875.

Though the PS theory is now widely accepted, various alternate explanations have been proposed over the years for how this ubiquitous symbol came into existence. One of the most popular came from libertarian philosopher and author Ayn Rand, who in her 1957 novel “Atlas Shrugged” included a chapter on the dollar sign, which she claimed was a symbol not only of American currency but of the nation’s economic freedom.

According to Rand, the dollar sign (written with two downward slashes instead of one) came from the initials of the United States: A capital U superimposed over a capital S, minus the lower part of the U. No documentary evidence exists to support this theory, however, and it seems clear the dollar sign was already in use by the time the United States was formed.