Doubled die errors are a type of mint-made error. There are a few famous examples of doubled die errors in US coinage, such as the 1955 doubled die cent (one of the most valuable mint-made errors in existence). A doubled die error is a type of hub-and-die error, rather than a strike error, as many assume. The error originates when the die itself is constructed – not when the coins are actually struck.
The process of making a die begins with a hub, which bears a raised (or “relief”) image of the design. In older times, the hub was modeled off of a plaster sculpture up to a foot in diameter, created by the coin’s designer (often a sculptor). Special machinery was used to reduce this large model down to the size of the actual coin on the master hub. In modern times, this process is accomplished with the aid of computer-controlled drilling machines and other technology. The master hub is used to create a number of master dies; since the hub (bearing the relief image) is pressed into the die, the die bears a sunken (or “incuse” image). These master dies are in turn used to create working hubs (relief), which themselves are used to create working dies (incuse). It is from these working dies (with the incuse image) that the design is struck onto a planchet (or “blank”), thus resulting in the coins themselves having a relief image. It is through this long and complex process that the final product bears an image similar to the one originally made by the designer.
Sometimes, in the process of transferring the relief image on the working hub to the incuse image on the working die, an error occurs. In older times, one impression was not enough to fully transfer the design from the hub to the die, so the hub and die came into contact more than once. If either the hub or die is even slightly misaligned on a subsequent strike, the die may receive a second incuse image offset from the first (hence, “doubled”). When this die is used to create coins, the coins themselves bear a corresponding relief image showing the multiple, offset strikes. Numismatists have identified eight classes of doubled die errors, each very specific to what went wrong: rotated, distorted, offset, pivoted, and so on.
The US Mint’s die making process was improved in the late 1990s. As a result of these technological improvements, the working dies only require a single impression to receive the image from the working hubs. Nonetheless, the immense pressure required to transfer the design in a single impression sometimes causes the die to rotate. Even though the hub and die only come into contact once, this is still considered a class of doubled die error. Some 2004-5 “Westward Journey” Jefferson nickels show signs of this doubling error.
True doubled die errors should not be confused with die deterioration doubling, another type of mint-made error. Die deterioration doubling, as the name might suggest, is caused by a problem with the die itself. It is possible that there was an error in the annealing process, a heat-treating procedure the dies undergo before striking. If there is not enough carbon present in the furnace, or if the dies are not cooled completely (ferrous metals, unlike other metals, must be cooled slowly), the die may be too soft. Die-deterioration errors may also occur when a die has simply been in use for too long. These errors are usually noticed first on the date and mintmark, which often appear alone in the “field” of the coin and are most susceptible. These details may appear “soft” and may give the impression of a doubled die error. Unlike doubled die errors, most die-deterioration errors are not valuable; these errors have occurred on almost every US coin series. Many unscrupulous dealers have attempted to pass off die-deterioration as much rarer doubled die errors, but savvy collectors should not be fooled.