Jamestown Issue - The Jamestown Exposition stamps were issued to promote the Jamestown Exposition of 1907 at what is now the Norfolk Naval Base in Virginia. more...

Jenny - Sometimes used a shortened nickname for the "inverted jenny", "Jenny" refers to the Curtiss JN-4 biplane that appears on the first three U.S. Air Mail stamps. The Curtiss JN-4 combined the best of both the "J" and the "N" models of the Curtiss airplanes. The term "Jenny" comes from this.
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Johl, Max - the author of the seminal works on the early 20^th century U.S. stamps. See also: the 1847USA Literature Page and click on "J".

Joint Line - the line produced from the ink that gathers in the separation where two rotary press plates meet on the printing cylinder. If the two plates are snug, the resulting joint line might be barely perceptible, if the gap is larger, the joint line could be quite thick to the point of producing what appears to be a double line. Joint lines are most commonly collected in pairs or strips. The coil pairs are known as "joint line pairs". Note that a joint line is NOT the same as a guide line, which was printed intentionally to guide the perforation process. 

Joint Line Pair - a rotary press coil pair with a joint line between them. Since joint line pairs command a higher premium than normal pairs, it is not uncommon to find joint line pairs with the line drawn in. Precaution must be taken when buying these and if the pair is expensive, a certificate of authenticity is advised.

Jumbos - stamps with large margins on all four sides, sometimes referred to as "with boardwalk margins".