The solidus replaced the Roman aureus in the early fourth century and became the standard high-value coin throughout the Mediterranean world for centuries. The reverse lettering CONOB means that this coin was struck at Constantinople, modern-day Istanbul, from refined gold, "obryzum". The letters OB may also represent the Greek number 72, as there were 72 solidi to the Roman pound.
The obverse of this coin shows a double portrait of Constans II alongside his son Constantine IV, whom he made co-emperor. Like many emperors, Constans met an unfortunate end as he was assassinated while taking a bath.