Before the advent of banks, the ancient Romans had a variety of ways to store their wealth. One of those ways was to bury it. That was particularly the case in the 3rd and 4th centuries as the empire started to decline. In the wake of barbarian invasion, civil war and economic collapse, wealthy people were forced to move and couldn’t carry all their coinage. So they left it behind, hoping to retrieve it later.
This means when coins are found, they are found in hoards, thousands of coins waiting in the ground for almost 2000 years. Some survive entirely intact, their inscriptions still readable. Some have been worn and corroded down to nothing.
Only a fraction of a percent have survived. But the empire lasted 500 years and was home to 50 million people. Billions of coins were in circulation at any one time, meaning millions of coins are still in existence.
Every Roman coin bears a portrait. Most commonly, it’s that of the current emperor. For many people, the change in coinage was how they learned there was a new emperor. But the coins bore other things as well. Sometimes the current emperor paid tribute to a previous emperor by depicting him on the coins. Sometimes they depicted family members, most commonly wives.
In the 1st and 2nd centuries, the portraits were incredibly detailed and unique. Emperors can be identified by their images. By the fourth century, the portraits had transformed into highly stylistic, nearly identical images. Some of these coins can still be identified by their reverse side, For others, identification depends on a surviving inscription.