The imperial denarius, the Roman coin for supra-regional trade, was devalued continually in the course of the 1st century AD. This meant that denarii had their silver content consistently diminished. The copper coins on the other hand, which were minted locally for every-day use, kept their value.
At a certain point the local authorities refused to exchange their good quality copper against the debased imperial silver coins at full rate. One denarius would have been equal to 16 asses or 4 sestertii; in practice, however, at the beginning of the 2nd century one denarius had a value of only one as.
The city authorities of Tium (in the province of Bithynia in Asia Minor) used a simple trick to sidestep the problem of the devalued denarii: They did not debase the value of the imperial coins, but increased the value of their copper coins. The coin shown here has a big countermark illustrating an H beneath the bust of Caracalla. This countermark doubled the value of the sesterce (originally four asses) to eight asses (H was the 8th letter in the Greek alphabet).
This coin is one of the very few known sestertii with a Greek countermark and therefore very rare.