Germany
Overview
In modern times there is a nation-state called Germany that occupies a large chunk of central Europe. That country did not exist in the Middle Ages. Instead, that part of Europe was made up of many various principalities—duchies, counties, margraviates, and so on—along with a couple of kingdoms. Ruling over all of them was an elected king who was elected with the title of King of the Germans, but who could also be named Emperor of the Romans. Towards the end of our period is the first recorded usage of the term "Holy Roman Empire" though historians often use the phrase in periods much earlier.
It was a peculiar institution, tracing a convoluted lineage back to Charlemagne. It was he who revived the title of imperator in the West (there had always been an "emperor of the Romans" in the East, ever since the time of Constantine). Charlemagne's empire was divided among his grandsons into three sections. The westernmost portion eventually became France, for the Kingdom of the Franks was even older than Charlemagne's empire. The other two portions, though, had a complicated history through the ensuing centuries. By the 10th century, it was established that the lords of the easternmost inheritance chose one of their number to be the King of Germany, and this fellow could also be crowned Emperor by the pope.
It's important to note that the King was elected and that the lords were more or less independent—he was an overlord more than a direct ruler. Through the 12th and 13th centuries, some kings were strong and ambitious and tried to assert their authority more directly. Others were weaker and were hardly more than first among equals.