During the Great Recession, the Federal Reserve lowered the federal funds rate nearly to zero and began using unconventional monetary policy. A fed funds rate near zero is no longer a proper representation of policy. Thus, empirical models of monetary policy cannot be estimated as usual. We use a linear empirical model to investigate whether alternative instruments such as the balance sheet or shadow rates can replace the fed funds rate to capture unconventional policy. Our objective is to determine whether adding to or replacing the policy instrument can preserve linearity or whether one must allow structural breaks. We include data for both normal and unconventional periods and find that shadow rates preserve linearity better than using a bounded federal funds rate alone, adding the balance sheet, or adding long rates. When short rates are bounded, shadow rates produce similar responses to the unbounded period and alleviate the need for structural breaks.