Carolingian Europe had minted only silver coins since the eighth century. Gold coinage was the preserve of Byzantium and the Islamic world: the solidus of Constantinople, the dinar of Baghdad. By the middle of the thirteenth century, however, Mediterranean trade, the Crusades, and Eastern goods (spices, silk, alum) had drawn extraordinary quantities of precious metal into the cities of northern Italy. In 1252 Florence, soon followed by Genoa and Venice, began striking its own gold coin. The Florentine florin (about 3.5 grams of pure gold) became within a few decades the international reference currency, used by merchants from the Atlantic to the Levant.

The gold coin was not just a change of metal; it was a sign of the credit system working behind it. In Florence the Bardi and Peruzzi families set up Europe's first genuinely multi-branch banks: London, Bruges, Paris, Avignon, Naples, Alexandria. A customer would deposit cash in Florence and, by presenting a bill of exchange (lettera di cambio), withdraw the equivalent in local currency in Bruges. The letter was both a foreign-exchange operation and a quiet instrument of interest — the gap between exchange rates concealed the 'usura' that the Church forbade. The mechanism freed trade from the risk of physically transporting gold.

Record-keeping changed too. By the fourteenth century the ledgers of Florence and Venice were standardising double-entry bookkeeping (partita doppia), in which every transaction was written twice — as a debit and as a credit. The method was only set down systematically by Luca Pacioli in 1494 ('Summa de arithmetica'), but the practice was already two centuries older. This deceptively simple innovation made it possible to see the state of a business at any moment, and it remains the backbone of modern accounting.

The political consequences were large. Bardi and Peruzzi lent to Edward III of England for the Hundred Years' War; when the king defaulted in the 1340s both banks collapsed, opening a generation-long depression in Florence. The vacuum was filled by the Medici, who in the fifteenth century became Europe's most powerful bank and the main patron of Renaissance art. The capitalist firm, the credit market, international banking, and sovereign debt — in their practical, working form — were all first assembled in these medieval Italian merchant republics.