Four emperors, one face
Diocletian answered fifty years of coups with an architecture of power: two Augusti, two Caesars, four capitals. The coinage became the chief instrument of that displayed unity.
The division
Emperor from 284, Diocletian took Maximian as his colleague for the West, then in 293 two Caesars, Constantius Chlorus and Galerius. Each had his zone, his residence — Nicomedia, Milan, Trier, Sirmium — and his share of the army.
The idea was to make usurpation pointless: power was already shared, the succession already provided for. The system held for some twenty years, until the abdication of 305, then collapsed into civil war.
A uniform coinage
The reform of 294 created the nummus, a large silvered bronze coin, and reorganised some fifteen mints from London to Alexandria. Everywhere, the same reverse: the Genius of the Roman People standing, modius on his head, patera and cornucopia, under the legend GENIO POPVLI ROMANI.
The most telling detail lies elsewhere. The portraits of the four princes become almost interchangeable: the same square jaw, the same thick neck, the same fixed gaze. Without the legend, one cannot always tell Diocletian from Maximian.
A political aesthetic
This is not clumsiness. It is the message: the tetrarchs are an institution, not four individuals. The porphyry group of the Tetrarchs set into St Mark's basilica in Venice says exactly the same thing in sculpture — four identical figures embracing.
Tetrarchic coinage is thus one of the rare cases where political doctrine can be read directly in the style of the portrait.
Further reading
S. Corcoran, The Empire of the Tetrarchs, Oxford, 1996.
RIC VI, Sutherland, for the classification of the mints after 294.