Did You Know: There are technically NO "over dates" in any U.S. coin series that begun after the 19th century. The term "over date" is an error/variety numismatic term to describe a coin with one date over another date (such as an 1829 bust half with the 9 over a 7), and that term was carried over by generalist numismatists to describe the more modern 1918/7 Buffalo Nickels and Standing Liberty Quarters and the 1942/1 dimes (just to mention a few), but technically this is not correct. The reason: Early in the history of the United States Mint each and every die was hand engraved and the lettering was punched or engraved into the surface of the individual dies. Every detail on every die was hand-made, so that when a date digit was placed over a different date digit, it was done more or less intentionally by someone with a punch. It could have been a quick way of making needed dies, it could have been an intentional thing done because they figured nobody would notice - there could be many reasons why the mint would do this, but the fact of the matter is that they KNEW they were doing it. By the time the coins of the 20th century came around (Lincoln cent, Buffalo Nickel, Mercury dime, Standing Liberty quarter, Walking Liberty half dollar, and Peace dollar), we were making far more coins, and the process of making dies was upgraded significantly to include a master engraving called a "master die" that already included the date. The master die was used to press the design into positive relief bars of steel called "master hubs", which were used in a pyramid style of multiplication to end up with the working dies which created the coins. Because the dies are made of hard steel, it was necessary to press the design into the die (or 'hub' the die) a number of times to complete the process. In between hubbings the die would go into an annealing oven to treat the metal for the next hubbing. Dime and cent dies often required 4 to 6 hubbings, where silver dollar dies often required 12 or more hubbings. The mistake that gave us the so-called "overdates" of the 20th century happened in the "hubbing" stage of creating dies. Basically a 1917 hub would be used to start the process of creating a die, then a 1918 hub would be mistakenly used to finish the process. The result would be a die that exhibited both dates to varying degrees - no punches, nothing hand-done, and it is VERY possible that the people making these dies might not have even known they were doing it. In the technical numismatic world, these are considered "class 3, design hub doubled dies". The definition of this class involves using two separate hubs to create a single die where the two separate hubs had differing designs. Another example of this is in the 1878 Morgan dollars where a tail feather design change shows both versions on a single die giving us the 8/7 tail feather dollars of 1878. Some of you may be familiar with the 1960 proof cents with both large and small date designs together on the same coin - again, a class 3 doubled die. In the world of collecting doubled dies, there are actually NINE different 'styles' of how a doubled die was created, each distinguishable by the result produced on the coins. For instance, a class 1 'rotated hub' doubled die explains the 1955 cents, with evenly spaced design elements all around the rim, which indicates a rotation in the position of a hub during the creation of the die that had an axis near the center of the design. Conversely, a class 5 'pivoted hub' doubled die explains the doubling on the well-known 1995 cent doubled die on which the doubling in LIBERTY is strong, but the date is hardly changed from 'normal'. This is because on a class 5 doubled die, the design is pivoted in one direction or another from a point along the edge of the design. Each of the nine classes is very different from the others, and a class 3 doubled die specifies a difference in the design from one hub used on a die and another hub used on the same die. So technically speaking - the 1918/7 and 1942/1 coins we are used to calling "overdates" - in fact are not, because the process that would have created an overdate (in its technical sense) were not used when these coins were created. So technically they are actually doubled dies.