DMPL is an acronym for "Deep Mirror Prooflike." It is used interchangeably with "DPL," which is used on NGC slabs to denote the same concept.
A DMPL coin features highly reflective surfaces that cast a clear reflection from many inches away. This is in contrast to a PL ("prooflike") coin, which has considerably less reflectivity. Different experts and/or grading companies have different definitions of the exact number of inches of perfect reflectivity required to justify a coin's designation as DMPL.
DMPL coins result from being struck by highly polished dies. The mirror-like quality resembles that of a proof coin, but a DMPL coin is not a proof coin. It is a business-strike coin intended for circulation and is therefore not proof but proof-like.
Since the first strikes of a die are often the most highly-polished strikes of a die's useful life, DMPL coins are often considered the "first off the presses." Thus, pitting and die gouges, which are usually created during the useful life of the die, are not typically associated with DMPL coins. Accordingly, many defective dies that did not meet their expected lifespan were only in use long enough to strike DMPL or PL coins. As such, many of the rarer die varities of specific dates and mints (notably 1878 eight tailfeather varieties) are often or usually found with reflective surfaces.
Additional Articles on the DMPL's
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