Since the early 1800's, letters have been used to identify where a coin was minted. "D" for Denver, "S" for San Francisco, and so on. On the Lincoln cent, the mint mark is located on the obverse side just below the date.
Until 1990, the mint mark was punched into the die by hand, many times they needed to use multiple strikes to get a good impression. If the punch shifted during one or more of the strikes, it would result in a double image of the mint mark. This second image could be out of line with the first, even rotated one way or the other. Sometimes it would be in a completely different area. Sometimes, if the punch shifted more than once, there would be a triple mint mark, or even a quadrupled mint mark. Some year Lincoln cents have many different RPMs. The 1960-D, for example, has over 100 different re-punched mint mark varieties.
In 2017, the 225th anniversary of the mint in Philadelphia, the mint issued cents bearing the "P" mint mark for the first time in it's history. The cents with the "P" mint mark are included in collector mint sets as well as business strikes issued for general circulation. Since the later mint marks are added directly to the master die, there is no chance of having a repunched mint mark.
When identifying which RPM you have, it is important to note the location of the mint mark in comparison to the date.
Sometimes a die will be punched with a mint mark and then be punched with a different mint mark over the first, such as an "S" over "D". These are know as over mint marks (OMMs).
Both RPMs and OMMs are desired by collectors.
Image by R.S.Cooper Edited By LCO
Click on a year below to see the RPMs for that year.