There was also a balance to weigh the coins to thier precise metal value. The mint, placed centrally in the room, consisted of an anvil fastened to a tree trunk and the die press. The anvil produced the obverse image, and the die press, the reverse. When the metal pellet was weighed and softened, the minter placed the metal on the anvil and then pressed the punch die on the reverse of the coin to imprint the second image. In order to reinforce the prints on both sides of the coin a hammer was struck on top of the punch die.
Errors are common since the ability of the workmen involved in making coins was of varying degrees.
A coin might be struck off-center or too weakly, or double struck. Around 420 BC Athens issued a decree that it had complete authority over all other Greek states to mint currency and to standardize weights and measures. This supremacy was later challenged by other Greek states which formed counter alliances. Indeed, the Boeotian confederacy was a rival minting alliance, that accepted the Aeginetic standards of weights and measures. The Boeotian league began in the 5th century BC as a political federation under the leadership of Thebes.