The start of what is viewed as Byzantine currency by numismatics began with the monetary reform of Anastasius in 498 AD, who reformed the late Roman Empire coinage system which consisted of the gold solidus and the bronze nummi. The nummi was an extremely small bronze coin (about 8-10mm), which were inconvient because a large number of them were required even for small transactions. The new bronze coins were made up of multiples of this coin such as the 40 nummi, 20 nummi, 10 nummi, and 5 nummi coins (other denominations were occasionally produced). The obverse (front) of these coins featured a highly stylized portrait of the emperor while the reverse (back) featured the value of the denomination represented according to the Greek numbering system (M=40,K=20,I=10,E=5). Silver coins were rarely produced.
The Byzantine monetary system changed during the 7th century when the 40 nummi (also known as the follis), now significantly smaller, became the only bronze coin to be regularly issued. Although Justinian II (685-695 & 705-711 AD) attempted a restoration of the follis size of Justinian, the follis continued to slowly decrease in size. In the 10th century so-called "anonymous folles" were struck instead of the earlier coins depicting the emperor. The anonymous folles featured to bust of Jesus Christ on the obverse and the inscription "XRISTUS/bASILEU/bASILE", which translates to "Christ, King of Kings" (see iconoclasm).
Later concave (cup-shaped) coins known as trachy were issued in both electrum (debased gold) and billon (debased silver). The exact reason for such coins is not known, although it is usually theorized that they were shaped for easier stacking.