The Holy Roman Empire was a highly decentralized state for most of its history, composed of hundreds of smaller states, most of which operated with some degree of independent sovereignty. Although in the earlier part of the Middle Ages, under the Salian and Hohenstaufen emperors, it was relatively centralized, as time went on the Emperor lost more and more power to the Princes. Late in the history of the Empire, in 1792, a Diet (congress or parliament) of the Empire was called.
This is the main page for the list of States which were part of the Holy Roman Empire, as alphabetized in the adjacent template, at any time within the empire's existence between 962 and 1806. In the 18th century the Holy Roman Empire consisted of over 1800 separate immediate territories governed by distinct authorities. In 1792 there were approximately 150 secular territorial rulers with the status of Imperial Estate.
Imperial abbeys were religious houses within the Holy Roman Empire which for some period during their existence had the status of Reichsunmittelbarkeit ("imperial immediacy"): that is, such houses were answerable directly to the Emperor and were thus sovereign territories (however small), independent of other lordships.