Russian gosudar’ ‘sovereign’: Нistory and etymology

In the first half of the 15th century, the word gospodar’ appears in the title of the Grand Princes of Muscovy. In the 16th–17th centuries, gospodar’ was replaced by gosudar’. How did these terms correlate? When exactly and how did this change take place? All researchers, without exception, believed that gosudar’ is the result of a transformation of the word gospodar’, but none of them could explain how it could happen. Meanwhile, these words, apparently, are not related at all: gosudar’ and gospodar’ have different origin, and one form cannot be received from the other. Gospodar’ is obviously connected with such words as Gospod’ ‘Lord’, gospodin ‘master’, etc.; but what is the origin of the word gosudar’? The article offers a solution to this question.