Karat plumb (also written "plumb karat" or "KP") is the United States trade designation for gold articles whose actual gold fineness equals or exceeds the marked karat fineness, with no permissible negative tolerance below the stamped value. A 14KP piece, for example, must assay at 583 parts per 1,000 (the mathematical fineness of 14/24) or higher, rather than at the older industry-tolerated minimum that allowed up to half a karat below the mark.

Origin and the 1976 Stamping Act amendments

The KP designation entered widespread US trade use in the late 1970s as a result of amendments to the National Gold and Silver Stamping Act of 1906. Prior practice in some segments of the United States jewellery industry had operated under a customary tolerance allowing actual fineness to fall as much as half a karat short of the stamped value, justified historically by the difficulty of working to exact fineness. The 1976 amendments effectively required that stamped karat fineness be the minimum actual fineness, eliminating the old half-karat tolerance; later FTC Jewelry Guides allowed a narrow technical tolerance of 3 parts per 1,000 (7 for soldered pieces) to accommodate manufacturing imprecision. The KP stamp emerged as a marketing differentiator for manufacturers signalling compliance with the stricter standard, even though after the amendment all legally compliant US gold jewellery was effectively plumb.

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Contemporary trade meaning

In contemporary US practice the KP suffix is now somewhat redundant from a regulatory standpoint — all gold sold into interstate commerce must meet the tolerances allowed under the FTC's Jewelry Guides at 16 CFR Part 23, which permit a negative deviation of up to 3 parts per thousand (7 for soldered pieces), rather than the stricter no-negative-tolerance plumb standard that KP represents. However, the suffix continues to appear on jewellery as a legacy marketing convention and a consumer-facing reassurance, particularly in the bridal and fine-jewellery categories. Internationally regulated markets such as India (BIS), the United Kingdom (Assay Offices) and the Hallmarking Convention member states have always required plumb fineness, and the KP designation is not used outside the North American trade.

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