How did the tetrarchy divide power among two Augusti and two Caesars?

The tetrarchy, established by Diocletian, was a system of power-sharing among four rulers designed to manage the vast Roman Empire more effectively and prevent usurpations.12

The power was divided into two tiers:

• Two Augusti: These were the senior rulers who held ultimate authority, made major decisions, and designated consuls. Diocletian served as an Augustus in the East, while Maximian served as an Augustus in the West.21
• Two Caesars: These were younger, secondary rulers who served in an executive capacity under the Augusti. Galerius was the Caesar attached to Diocletian, and Constantius I Chlorus was the Caesar attached to Maximian.21

Although the empire was divided administratively into four sectors—with each ruler assigned specific regions and troops—it remained a patrimonium indivisum (undivided inheritance). To ensure unity and loyalty, the Augusti adopted their respective Caesars, and the relationships were further solidified through strategic marriages. Diocletian maintained overall precedence, with the entire system functioning as a collegium where he directed the primary governmental decisions.12