In the context of national analyses, the irrevocably fixed exchange rate is often also presupposed for the past in order to avoid breaks in time series. However, this procedure is problematic if historical data of different countries are converted into euro and then aggregated or compared among themselves. It is thereby incorrectly assumed that the exchange rates between the national currencies were already fixed in the past. This may result in a distorted representation of the interactions between real and monetary processes within and among the economies.