For ancient coins, the classical numismatist traditionally uses the term reverse to describe the side of the coin struck with the die that was hit with the hammer. For the most ancient coins this side is easily identified because it was a plain punch and later a design within an incuse created by the punch.
The mobile or reverse die tended to wear out faster than the obverse die, which was fixed in the anvil. For this reason, the more important design was usually put on the obverse. For Roman imperial coins the most important design was, of course, the portrait of the emperor; hence the modern universal acceptance of the "heads" side of the coin as the obverse.
Dictionary of Roman Coins
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