1st March » Emperor Diocletian and Maximian appoint Constantius Chlorus and Galerius as Caesar (title) called Caesars. This is considered the beginning of the Tetrarchy, known as the 'Quattuor Principes Mundi' ("Four Rulers of the World").
21st May » Roman Emperors Diocletian and Maximian appoint Galerius as 'Caesar (title) or Caesar' to Diocletian, beginning the period of four rulers known as the Tetrarchy.