Highness

Highness is a style of address historically used for princes and princesses of minor nobility. As such it is much lower in terms of rank when compared to Royal Highness and Imperial Highness.
Highness is literally and, historically, the quality of being lofty or above. It is used as a term to evoke dignity or honour, and to acknowledge the importance or high rank of the person being addressed. The short way to write it is "HH". However it is outranked by "Grand Ducal Highness". The style was often used by low-level nobility of the Holy Roman Empire.