What is meant by double die in reference to US pennies?

Answer 1

A doubled die is a condition normally not found with modern practices found at the United States Mint; other countries' mints, as well as private mints, may still produce doubled dies. A doubled die is when a double image of the coin is created on the die used to strike the coin. In years past a master die was made by copying a plaster copy of the final coin to steel, reducing the size to the correct size of the coin in the process. This master positive would then be struck into another piece of steel, producing a die. That die would then be used to produce more steel positives, which would be used to create more dies, until all the dies that would be needed would be created. (The steel was hardened along the way to allow each result to strike more than one copy of die or positive image.) Because the force required to transfer the image was more than earlier equipment could handle, each positive or die would be struck multiple times. If anything changed between strikings, a double image would be created and transferred in subsequent strikings, eventually to the final coins produced by the doubled die or set of doubled dies. In some cases, unused dies from the previous year would be modified, replacing the previous year with the current one; if the previous image was not completely obliterated, a doubled die condition would also occur. A doubled die is not the same as a doubly-struck coin. Many coins are actually struck multiple times, either intentionally (in the case of proof coins) or unintentionally (such as die bounce).