Medieval coin plays an essential role in the imagined history of money: it figures as the primal "commodity money" — a natural medium, spontaneously adopted by parties in exchange who converge upon a metal like silver to represent the value of other goods. As a natural medium with a price objectively established through trade, commodity money appears to offer an independent means of measure in the market. But as the history offered here reveals, medieval money was nothing like its imagined alternative. England’s early coin became a medium when the government began to spend and tax in that unit of account, took coin as a mode of payment, and allowed it to be transferred between people in the meantime. Individuals participated in the arrangement, paying for coin in exchange for the unique quality — liquidity — that set money apart from a commodity. That quality was orchestrated by the very channels that brought public and private together in the project of making a medium. In fact, insofar as the English equated money with the commodity it contained, they engineered instability into the heart of their medium. Depreciating coin — diluting its commodity content — offered a cure. It also confirmed that coin had never been the "commodity money" imagined in later accounts. Coin was, instead, a constitutional medium, one that related the government to its participants and thus helped to configure the world it appeared merely to measure.