History of the Roman Empire
Theodosius was the last Emperor who ruled over the whole Empire. After his death in 395, he gave the two halves of the Empire to his two sons Arcadius and Honorius; Arcadius became ruler in the East, with his capital in Constantinople, and Honorius became ruler in the West, with his capital in Milan and later Ravenna. The Roman state would continue to have two different emperors with different seats of power throughout the 5th century, though the Eastern Romans considered themselves to be the only ones who were fully Roman. Latin was used in official writings as much as, if not more than, Greek and the two halves were nominally, culturally and historically, if not politically, the same state.
Holy Roman Emperor
From an autocracy in Carolingian times (AD 800–924) the title by the 13th century evolved into an elected monarchy chosen by the prince-electors.
Various royal houses of Europe, at different times, became "de-facto" hereditary holders of the title, notably the Ottonians (962–1024) and the Salians (1027–1125). Following the late medieval crisis of government, the Habsburgs kept possession of the title without interruption from 1440–1740. 
The final emperors were from the House of Lorraine (Habsburg-Lorraine), from 1765–1806.
The Holy Roman Empire was dissolved after the defeat at Austerlitz by emperor Francis II, who continued to rule as Austrian emperor.