Where do you draw the line for a common monetary policy? Would regions like Madrid or Barcelona, easily at e.g. Dutch level, have to share monetary policy with

Where do you draw the line for a common monetary policy? Would regions like Madrid or Barcelona, easily at e.g. Dutch level, have to share monetary policy with Andalucia or Extremadura - two of the poorer regions in Europe? Are there studies pointing to when or where a common monetary policy stops making sense?