what is "karat plumb"

You may come across a piece of jewelry marked 14KP.  At first blush many think this is 14 karat plated.  But this is not the case.  

The KP stands for "Karat Plumb." Obviously the Karat refers to the purity of the gold, while the plumb can be considered to mean 'exact', in the same way that a 'plumb' wall is exactly vertical or a 'plum' surface is exactly horizontal.

In gold bullion terms it means that the gold stamped with the KP is guaranteed to have a gold purity not less than indicated but could possibly be slightly more pure. So, for example, a bracelet stamped 14KP is guaranteed to contain 58.5% gold or more. Another bracelet stamped 585KP is also guaranteed to contain 58.5% pure gold or more. The KP stamp is designed to eliminate the non standard caratage tolerances that some markets allow for. In the USA, for example, gold can be sold at up to 0.5 carat more than it's actual gold content. So, a ring marked 14K can contain just 13.5 carats of gold. Furthermore, even though the USA only allows jewellery to be sold as gold with a minimum of 10 carats purity, the 0.5 carat leeway still applies and 9.5 carat is the actual minimum. So, as a consumer you'd be better off purchasing a ring stamped 10KP than a ring stamped 10K.