The first co-ruler. Maximian's co-emperor. His reign ended with his abdication.

CAESAR CAIVS AVRELIVS VALERIVS DIOCLETIANVS AVGVSTVS

Emperor Diocletian reigned from 284 to 305, during which time he made significant changes to the coinage system of the Roman Empire. The most important change was that Diocletian introduced two new coin types: the gold solidus and the copper follisos. The solidus was equal to 25 denarius and became the new main money of the empire. And the follisos was a cheaper coin that was used in everyday trade. Diocletian also introduced a new issue system in which the coins were marked with symbols indicating the place and date of minting. This system allowed monetary historians to more accurately determine when and where each coin was struck. Some examples of Diocletian's coinage:Gold solidus: The solidus was a large 24 carat gold coin equal to 25 denarius. During the reign of Diocletian, the solidus became the main currency of the empire and remained so until the end of the 4th century. Copper follisos: The follisos was a small copper coin equal to 1/20 of a solidus. The follisos was a cheap coin used in everyday trade. Diocletian's coinage had a significant impact on the monetary history of the Roman Empire. The solidus provided a solid financial foundation for the empire, and the follisos allowed the populace to have cheap access to money.