Actually, neither is really analogous to an individual credit limit. As you correctly point out, the debt ceiling is an entirely voluntary constraint that the

Actually, neither is really analogous to an individual credit limit. As you correctly point out, the debt ceiling is an entirely voluntary constraint that the US government imposes on itself (if, as is appropriate from an economics point of view, you take the government to be the union of executive, legislature and judicative branches). The interest rate for short-term T-bills necessarily follows very closely the monetary policy targets set by the Fed. In fact, the Fed and Treasury cooperate all the time to control the supply and demand of T-bills, in order to make sure that the interest rate established by the market matches the target rate - that's just part of normal day-to-day operations, see e.g. http://pragcap.com/resources/understanding-modern-monetary-system http://pragcap.com/resources/understanding-modern-monetary-s... Long-term interest rates behave slightly differently, and some people believe that useful information about the state of the economy can be extracted from them. This is still not analogous to a credit limit, because those long-term rates can never actually constrain the US government financially. After all, Fed and Treasury can always work together to reduce the supply of long-term bonds, which will drive down the interest rate.