 In addition, under Michael IV the Paphlagonian (), it began to be minted in a slightly concave (scyphate) form, possibly to increase the thin coin's strength and to make it less easily bent.Flat coins were still struck at times, but scyphate ones came to predominate from Constantine IX () on and became standard under Isaac I Komnenos ().These concave coins were known as "histamena trachea" or simply "trachea" (τραχέα, "rough, uneven") from their shape.