The dollar sign looks like the purest emblem of American capitalism, yet its origins have nothing to do with the United States. The popular story that it is a U laid over an S, promoted by Ayn Rand in Atlas Shrugged, has no paper trail behind it. Historians point instead to merchants’ ledgers, where the plural of the Spanish-American peso was abbreviated as a p with a superscript s. Written thousands of times a day with quill pens, the two letters slowly merged until only an S with a vertical stroke remained.

This episode follows the symbol from 18th century counting houses and the silver coins of Potosí to a Philadelphia type foundry and, finally, the computer keyboard. Along the way it weighs the coin imagery that reinforced the shape, discards several colorful myths, and shows how programmers repurposed a character that existed only because early business machines needed it for accounting.

• A 1778 letter by Oliver Pollock, an Irish trader and financier of the American Revolution, shows a merged ps nearly identical to the modern sign.
• Spanish pieces of eight showed the Pillars of Hercules wrapped in an S-shaped banner, and merchants in Qing dynasty China called them double pillar dollars.
• The Coinage Act of 1792 modeled the U.S. dollar on the Spanish milled dollar, and foreign coins remained legal tender until 1857.
• Philadelphia printer Archibald Binny cast the first metal type of the symbol in the 1790s, while the Portuguese cifrão used the same shape as a decimal separator.
• In BASIC the sign marks a text string, in Excel it locks a cell reference, and in nuclear physics one dollar is the threshold of prompt criticality.