Doubled Die Coins
The coin die that strikes double died coins has a partial or fully doubled image on it. Therefore, the coin has two identical images that are slightly offset. The doubling occurs from mistakes in the die hubbing process. This results in a coin die having more than one image on it.
1955 doubled die Lincoln Cents (see photo) are the classic example of a doubled die. These double died coins sparked the start of the error coin collecting hobby in the United States. Although no other doubled die types exhibiting the remarkable degree of doubling seen on the 1955 Lincoln cent, other cents have emerged from the U.S. Mint. Other noteworthy doubled die coins are the 1972 and 1995 Lincoln Cents.
Not Double Struck
Many people mistake double struck coins for doubled die coins. The key difference is that double struck coins are struck more than once while the coin is in the coining chamber of the coining press. When this happens, the first impression is flattened, or sometimes obliterated by the second strike. Subsequent strikes will also flatten or obliterate the design from earlier strikes.
Coin dies that are not appropriately made result in doubled die coins. As indicated above, it takes several impressions from the coin hub to make a coin die. If the mint employee does not align the coin hub perfectly above the coin die, a second impression will result on the coin die that is slightly off center from the first. If this coin die is used to make coins, all coins made from the die will have this doubling effect.
Doubled Die Designations
Coin collectors and numismatists will examine the coin indicating if the doubling on the obverse or the reverse. Some doubling effect is so minute that it requires a loupe or microscope to see it. When cataloging coins, coin collectors and numismatist will designate Double Die Obverse with the letters "DDO." Double Die Reverse coins will carry the "DDR" designation.
Tripled, Quadrupled and More
Since the manufacturing process to produce a coin die involves multiple impressions from the coin hub, it is possible that more than two impressions may be needed. If the mint worker does not precisely align the coin hub with the coin die, it is possible to produce a Tripled Die or a Quadrupled Die. These are designated as follows:
Beware of Chinese Counterfeits and Machine Doubled Coins
Chinese counterfeiters are manufacturing some of the more valuable doubled die coins (1955 and 1972 Lincoln pennies). These are high-quality coins made by counterfeit manufacturers in China. Unfortunately, it is not against the law in China to make "reproductions" of United States coins. Therefore, before you buy one of these high-priced coins, you should make sure that you are buying it from a reputable coin dealer or purchase a coin certified by a third party grading service.
A second confusing aspect of doubled die coins is that some people confuse them with machine doubled coins. This error type is also known as Mechanical Doubling or Die Abrasion Doubling. These coins are technically considered mint errors, but they are not collectible and are only worth face value.
The proper numismatic term is "doubled die." Sometimes people refer to these as "double die." Although this is incorrect, most coin dealers will know that you are referring to doubled die coins.
Edited by James Bucki