The theory of the mechanics of coin striking is derived. From this it is shown how energy of striking is related to the coin geometry. The striking energies for a series of Byzantine solidi were calculated based on a simplified geometry for the coin. The results show that the striking energy would have had to increase substantially, because of alloy changes, unless the coin size and method of striking had not developed to minimize this. The analysis indicates why distorted (concave) coins were produced. -- AATA