Early coins were made by hand, one at a time. To produce a coin, the artisan first heated a small metal disc (or blank) and placed it between two carved molds (called dies). He then hit the mold with a hammer, striking the design on to both sides of the coin.
This process had plenty of room for error. Blanks were irregular in shape and mass and the molds wore out differently with repeated use. As a result, no two coins look exactly alike. Nonetheless, merchants would happily accept any silver coin, even if imperfect.