Aftermath of Rome's fall
With the fall of the Western Roman Empire, cities and towns shrank or disappeared altogether. Long-distance trade and a sophisticated monetized economy were replaced by barter and an agricultural subsistence economy. These changes are reflected in coinage.
The Western Roman Empire was replaced by a patchwork of barbarian kingdoms, isolated domains of Roman landlords and simple chaos. Money became scarce and was increasingly limited to one of two denominations in gold, except for in Italy, where a monetized economy survived due to the proximity of the Eastern Roman Empire.
Over the course of the 7th century conditions began to settle down as the barbarian tribes formed permanent territorial kingdoms. The economy began to revive and coinage was struck in greater numbers.
As Western Europe recovered, limited coinages of gold fractions were replaced by silver "pennies" based on the Roman denarius-these coins became the basis for the monetary system introduced by Charlemagne.
The Eastern Roman Empire had troubles of its own. After a short military and economic revival in the 6th century, the empire was faced with a series of devastating wars with the Sassanid Persian Empire based in Iraq and Iran.
The Byzantines emerged victorious just as the Arabs under the prophet Mohammed erupted from southern Arabia with their new religion, Islam. The Arabs were able to defeat the weakened Byzantines and Sassanids, creating a new empire (the Caliphate) which spread from Spain to Afghanistan and beyond.
The Caliphate developed its own coinage, in keeping with Islam beliefs, which replaced its Byzantine and Sassanid predecessors as the standard trade coinage of western and central Asia.