Other Forms of Doubling:
With little doubt, for new variety and error collectors of any denomination of coins, both foreign and US produced, the identification and understanding of Other Forms of Doubling is critical.

True doubling, and phenomena that resembles doubling, have many causes. Of what is encountered, Machine Doubling (MD) and Die Deterioration Doubling (DDD) are the most commonly encountered and misunderstood by new collectors and even long time collectors who never took the time to properly study them. Master Die Doubling, Obverse (MDO) or Reverse (MDR), are also commonly found, in particular on the 1972 Lincoln cent.

Collectible doubling is related to a few specific forms of true doubling. Die Variety Doubling: Hub Doubling (i.e., Doubled Dies); Repunched mintmarks (RPM); Repunched Dates (RPD). Die Errors: Double (or Multiple) Strike; Counterclash; Ejection Impact Doubling (EID) and Rim-restricted Design Doubling [M. Diamond, Errorscope Vol. 17, No 4, July, 2008]; along with other forms of Error Doubling.