In theory, the solidus was struck from pure gold, but because of the limits of refining techniques, in practice - the coins were often about 23k fine (95.8% gold). In the Greek-speaking world during the Roman period, and then in the Byzantine economy, the solidus was known as the νόμισμα ("nomisma", plural "nomismata"). In the 10th century Emperor Nicephorus II Phocas (963–969) introduced a new lightweight gold coin called the "tetarteron nomisma" that circulated alongside the solidus, and from that time the solidus ("nomisma") became known as the ἱστάμενον νόμισμα ("histamenon nomisma"), in the Greek speaking world. Initially it was difficult to distinguish the two coins, as they had the same design, dimensions and purity, and there were no marks of value to distinguish the denominations. The only difference was the weight. The "tetarteron nomisma" was a lighter coin, about 4.05 grams, but the "histamenon nomisma" maintained the traditional weight of 4.5 grams. To eliminate confusion between the two, from the reign of Basil II (975–1025) the solidus ("histamenon nomisma") was struck as a thinner coin with a larger diameter but with the same weight and purity as before. From the middle of the 11th century, the larger diameter "histamenon nomisma" was struck on a concave flan, while the smaller "tetarteron nomisma" continued to be struck on a smaller flat flan.