I'm not sure what you mean by DD, do you mean Doubled Die (Hub Doubling) or a Double Strike? A Double Strike is easy as it is creating by two (or more) separate strikes from a die, whereas MD is created by some of the extra energy from a single strike where the Die Face bounces or vibrates up and down or in some rare cases drifts sideways.
A Doubled Die is created from the Hub, which is a positive (and looks like a struck coin), that has been misaligned in a subsequent hubbing (or moved during a single hubbing - post '97). The positive parts of the Hub stick out and create impressions (incuse) into the Die Face; in essence holes or cavities replicating the design. In the hubbing process once you create an incuse area, adding to that incuse area within the area of the fields always makes the design a little larger. You can experiment with this by making impressions of a coin in clay (or play-dough) and then turning or offset the coin and making another impression.
I hope I am answering your question...
A Doubled Die is created from the Hub, which is a positive (and looks like a struck coin), that has been misaligned in a subsequent hubbing (or moved during a single hubbing - post '97). The positive parts of the Hub stick out and create impressions (incuse) into the Die Face; in essence holes or cavities replicating the design. In the hubbing process once you create an incuse area, adding to that incuse area within the area of the fields always makes the design a little larger. You can experiment with this by making impressions of a coin in clay (or play-dough) and then turning or offset the coin and making another impression.
I hope I am answering your question...
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