PART VI. Striking Errors:

Abnormally Strong Strike:

Finning

Definition: A fin is a thin flange that extends vertically from the rim/edge junction of a coin struck within the collar.  It is caused by excessive striking pressure or a localized increase in striking pressure caused by slight die tilt.  The extra pressure causes coin metal to squeeze into the narrow gap between die neck and collar.  Fins can develop on either face or both faces simultaneously.  A fin may encircle a coin or may be restricted to one pole.  The latter is the kind typically produced by die tilt.

The 1993-P Lincoln cent pictured above shows a finned rim on the reverse of that coin (see black arrows). This portion of the rim is extended past the normal height of the remainder of the rim.