Coins are made by minting machines that apply tremendous stamping pressure to pieces of metal, leaving artistic patterns in the finished coin. The piece of metal, or coin blank, is called a planchet, and the hardened patterns that stamp the coin are called dies. There are two dies in a minting press, one die for the front, and one die for the back of the coin.
After hours and hours of coin striking, the dies sometimes break under the pressure. Mint workers usually replace the dies before they crack, but sometimes several coins are produced before the workers find and replace the dies. If the broken dies go unattended for long periods of time, the break grows worse and worse, leaving larger and larger malformations on the struck coins. A large lump of coin metal left by a major die break is called a cud or sometimes extra metal.
The cud on the Licoln cent in our picture is quite spectacular. If you look closely, you can see the effects of the die break on the reverse (tails side) of the coin. This is such a remarkable error, it would probably sell for $50 to $100 retail. A dealer would usually pay one-half that amount. If the coin were a Washington quarter or a Mexican peso, the value would be about the same. Collectors are interested in the malformation, not the denomination of the coin. Less spectacular errors are worth less.
CoinQuest thanks respected eBay seller frosty444 for use of their coin image. It is a beautiful example.