An overstrike is a coin struck (overstruck) on an older coin, where the the older coin was used as the "blank" flan. Often some of the legend or type on the older coin is still visible. In addition there are usually undertype effects on the new coins design. Due to the uneven surface of the undertype, parts of the die may not fill, leaving some elements of the new type unstruck.

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de Callataÿ, F. "A Coin with the Legend ΘPAKΩN Overstruck on an Athenian Stephanophoros Tetradrachm of AΠEΛΛIKΩN-ΓOPΓIAΣ (88/7 BC) and its Consequences for the Thasian Type Coinage" in Studies Prokopov.
Emmons, B. "The overstruck coinage of Ptolemy I" in ANSMN 6 (1954), pp. 69 - 83.
MacDonald, D. Overstruck Greek Coins: Studies in Greek Chronology and Monetary Theory. (Atlanta, 2008).
Rosenberger, M. The Rosenberger Israel Collection Volume IV: The Coinage of Eastern Palestine, and legionary countermarks, Bar-Kochba overstruck. (Jerusalem, 1978).
Southerland, C. "'Carausius II','Censeris', and the Barbarous Fel. Temp. Reparatio Overstrikes" in NC 1945.
Stannard, C. "Overstrikes and imitative coinages in central Italy in the late Republic," in Essays Hirsch. (1998)

Overstrike

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