State-contingent government bonds have long been championed as a way to make sovereign borrowing safer, but the cost for borrowers has remained difficult to measure. This column discusses how newly hand-collected daily bond price data from a forgotten postwar French experiment and quasi-twin bond estimation strategy can be used to estimate the state-contingent debt premium. By comparing a conventional bond with a state-contingent twin linked to industrial production, it shows that expected and realised premia during normal times are an order of magnitude smaller than those estimated from restructuring-era instruments. State-contingent debt may not be as expensive as previously thought. […]