The dollar sign hides a small chronological joke: the symbol is very likely older than the currency it now names. The leading theory of its origin points not to the United States but to Spanish America, where clerks and merchants wrote the abbreviation ps for pesos over and over in their correspondence. Written at speed and in quantity, the two letters began to overlap. The stem of the p came down across the s, again and again, until the pair merged into a single mark, the vertical stroke through the curved line that we now read at a glance as money.

The documentary trail supports the surprise. The symbol appears in manuscripts from the 1770s, its form already recognisable, standing for the peso in the hands of writers who had never heard of an American dollar. Only afterward did the United States adopt its own currency, a dollar descended from that same peso, and inherit the sign along with it. So by the time the mark came to mean the dollar, it had already been in use for years to mean something else. The country did not invent the symbol for its money; it borrowed a symbol that was already busy naming another coin.

This reversal is exactly the kind of thing the exhibit delights in. We tend to assume a currency's symbol is coined alongside the currency, a fresh mark for a fresh money. The dollar sign shows the opposite can be true, that a symbol can outlive one currency's dominance and be handed on to the next, carrying its worn-in familiarity into a new role. Every time the sign is written today, in front of a price or a sum, it reaches back past the dollar to the peso, and past deliberate design to the ordinary friction of a clerk's pen writing ps too many times in a row. The character is a fossil of habit, a shortcut that hardened into a symbol before the money it now serves had even been named.