Doubling

kmaldona,

"Doubling" is a term used to describe a coin that exhibits the appearance of a second image of part or all of its features, one image slightly misaligned from the other.
Doubling can be caused by the movement of the coin machinery, by Die Deterioration, or by the double hubbing of the die.

"Doubled Die" happens when the die itself is doubled. If a die requires more than one hubbing, and the second hubbing is misaligned from the first hubbing, then the die itself is doubled, and every coin that die strikes will be doubled, hence "Doubled Die".
If the misalignment is significant, the result is strikingly obvious, as in the case of the 1955 doubled die Lincoln Cent.
Most doubled dies are not that obvious and require close examination to detect.

Hope this helps,

Steve