Plumb Mark
Plumb Mark
The P stamped after a karat number on American gold, signalling exact fineness with no tolerance taken
The plumb mark is the P suffix stamped after the karat number on American gold jewellery — most often as 10KP, 14KP, or 18KP — indicating that the alloy meets its stated fineness exactly, without the downward tolerance the Federal Trade Commission once permitted. The mark reached its peak use in the mid-to-late twentieth century as a voluntary quality-assurance signal and has been progressively superseded by tighter regulatory tolerances that now apply by default to all karat-stamped American gold.
How to read the mark
A plumb mark appears immediately after the karat number with no space, and is distinct from a manufacturer's trade mark or the mandatory United States responsibility mark. A ring stamped 14KP JR-CO might read in full as 14-karat plumb gold, manufactured by JR-CO, with the karat designation and the responsibility mark together forming the legally required information. The plumb portion is voluntary; the karat number and the responsibility mark are not.
On older American work, the mark sometimes appears as 14K Plumb spelt out rather than as the P suffix. The two forms are equivalent. The mark is specific to gold; there is no equivalent plumb sterling mark in common silver use, although the term is sometimes loosely applied to sterling that meets or exceeds 925 fineness.
Current relevance
Modern American hallmarking law has effectively closed the gap between plumb and non-plumb karat gold. The FTC's Jewelry Guides require fineness to fall within a narrow analytical margin, and the older half-karat tolerance for unsoldered articles has been removed. New manufacturing rarely applies the P suffix because the distinction it once carried no longer exists. Most contemporary American karat gold is marked simply 14K or 18K and is, in regulatory terms, plumb gold by definition.
In the trade
For appraisers, the plumb mark is principally a dating cue. Its presence suggests twentieth-century American manufacture from the period when the distinction was actively marketed. The mark itself adds nothing to current value; the underlying fineness, the maker, the design quality, and the condition are what matter. Estate buyers occasionally cite the P suffix as a quality assurance, but the assurance it once carried is now baseline regulation.