What is "histamenon"

Histamenon (, "standard coin") was the name given to the gold Byzantine solidus when the slightly lighter tetarteron was introduced in the 960s. To distinguish the two, the histamenon was changed in form from the original solidus, becoming wider and thinner, as well as concave ( scyphate) in form. Later usually shortened to stamenon (Greek: στάμενον), it was discontinued after 1092. In the 12th and 13th centuries, the name stamenon came to be applied to the concave billon and copper trachea coins.