The title ' (female form ', female plural ') is used for the heads of princely houses of German origin (in German a '). From the Late Middle Ages, it referred to any vassal of the Holy Roman Emperor ruling over an immediate estate. Unless he also holds a higher title, such as grand duke or king, he will be known either by the formula "' + origin of the dynasty", or by the formula "' + of the ruled territory". These forms can be combined, as in """".