The application of exchange rate target zones modeling to interest rates allows interpreting the puzzles that emerged with the public debt euro area crisis, namely the nonlinear behavior of the interest rates and the fact that some stand-alone countries, not belonging to the euro area, have not been subject to speculative attacks in spite of equally large public debt-to-gross domestic product (GDP) ratios. As a matter of fact, this model shows that in the case of a noncredible upper threshold for the interest rate (that may be due to both the lack of room for increasing further the required government primary surplus and/or the absence of a monetary authority acting as a lender of last resort), the resulting public debt unsustainability determines an interest rate nonlinearity and makes the crisis possible for public debt levels that would be stable in the presence of a credible interest rate target.
