Thank you for your answer. I am reading the article on Hessen-Darmstadt (redirects to it) and it says "In 1871, it became a constituent state of the German Empire." So that's not sovereign anymore, anything before that until 1806 maybe, but not after the founding of the German Empire, all state monarchies lost their sovereignty. Here is also a list of all the states Category:States of the Holy Roman Empire. IMO they only had sovereignty between the end of the Holy Roman Empire from 1806 until the foundation of the Second Reich in 1871. The only monarchies that retained their sovereignty were Austria-Hungary, Luxembourg, and Liechtenstein, all others were incorporated and became mere states of the German Empire.