What was the Capital of the Holy Roman Empire?

There was no fixed capital, only residences of the German kings and emperors like Magdeburg (under the Ottonians), Speyer (Salians), Prague (Luxemburg), Vienna (Habsburg); and some important cities like Aachen (where the king was crowned), or Regensburg (where the Reichstag was) or Nuremberg (where the imperial regalia was kept).