Mint marks and exergues
Beneath the reverse design runs a band, the exergue, carrying a few letters. They tell you where the coin was struck, and often by which workshop within the city.
Reading the exergue
The marks generally combine an abbreviation of the city with a letter for the officina. PTR reads prima Treveris, the first officina of Trier; SMN, sacra moneta Nicomediae, the sacred mint of Nicomedia. Officinae are numbered in Latin or Greek letters according to West or East.
On reformed aureliani the mark XXI appears, sometimes KA in the East. It states the official ratio between the new silvered bronze and the silver coinage, and marks an issue later than the reform of 274.
The network of mints
At the beginning of the period, Rome strikes almost everything. The crisis of the third century scatters production: Milan, Siscia, Serdica, Cyzicus, Antioch, Lyon for the West. The tetrarchic reform organises that network, from London to Alexandria, with some fifteen mints in operation.
This dispersal is a gift to the collector. One and the same reverse type exists struck in several cities, with markedly different portrait styles. Many collections are built this way, by mint rather than by emperor.
Style as evidence
Every mint has its engravers and its habits. Balkan portraits are dry and angular, those of Rome rounder, those of Antioch recognisable by their treatment of the beard. With practice, style tells you something before you have even read the exergue, and lets you spot a doubtful attribution.