Augustus, the first Emperor, had nominally shared power with his colleagues, and more formal offices of Co-Emperor had existed from Marcus Aurelius on.
Most recently, Emperor Carus and his sons had ruled together, albeit unsuccessfully.
The first time Diocletian's whereabouts are accurately established, in 282, he was made by the newly Emperor Carus commander of the Protectores domestici, the élite cavalry force directly attached to the Imperial household – a post that earned him the honor of a consulship in 283.
He raised his sword to the light of the sun and swore an oath disclaiming responsibility for Numerian's death.
He asserted that Aper had killed Numerian and concealed it.
He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens.
His palace eventually became the core of the modern-day city of Split in Croatia.
Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively.