Currency Codes and Quote Direction

Every actively traded currency has a standard three-letter code defined by ISO 4217 — USD for the US dollar, EUR for the euro, JPY for the Japanese yen, GBP for the British pound — used universally in quotes, contracts, and settlement instructions instead of ambiguous symbols like "$", which several countries use for their own, different, currencies.

An FX quote always names two currencies as a pair, and the order matters: in "EUR/USD," EUR is the base currency and USD is the quote currency, and the quoted price tells you how many units of the quote currency one unit of the base currency buys. A quote of EUR/USD = 1.10 means one euro buys $1.10. Flip the pair to USD/EUR and the quote inverts to roughly 0.909, meaning one dollar buys about €0.909 — same market, same information, but a different number because the base and quote roles swapped.

Market convention, not logic, decides which currency is listed first for a given pair: the euro is conventionally the base against nearly every other currency (EUR/USD, EUR/GBP, EUR/JPY), and the British pound is base against the dollar (GBP/USD) by long-standing convention predating the euro. A newcomer reading "USD/JPY = 150" who assumes dollar-per-yen rather than yen-per-dollar will be off by a factor of over 20,000 — exactly the kind of mistake that ISO codes and consistent base/quote conventions exist to prevent, provided the reader knows which one is base.