Tyre bead profiles / rim contour designations.
J, JJ, K, JK, B, P and D
No, my keyboard letters weren't stuck when I typed this The letter(s) that typically sit(s) between the rim width and diameter (or vice versa) figures stamped on the wheel, and indicates the physical shape of the wheel where the tire bead meets it. (i.e 15x6JJ)
Like so many topics, the answer as to which letter represents which profile is a long and complicated one Common wisdom has it that the letter represents the shape. ie. "J" means the bead profile is the shape of the letter "J". Not so, although "J" is the most common profile identifier. 4x4 vehicles often have "JJ" wheels (like the photo above).
Jaguar vehicles (especially older ones) have "K" profile wheels. Some of the very old VW Beetles had "P" and "B" profile wheels.
Anyways, the reason why it's an "awkward topic to find definitive data on" is very apparent if you've ever looked at the 'Standards Manual of the European Tyre and Rim Technical Organisation' manual. It is extremely hard to follow...
There are pages and pages (64 in total) on wheel contours and bead profiles alone, including dimensions for every type of wheel you can think of (and many of you probably can't) with at least a dozen tabled dimensions for each. Casually looking through the manual is enough to send you to sleep and
looking at it with some concentration, is enough to make your brain run out of your ears
To try to boil it all down for you, it seems that they divide up the rim into different sections and have various codes to describe the geometry of each area. For example, the "J" code makes up the "Rim Contour" and specifies rim contour dimensions in a single category of rims called "Code 10 to 26 on 5deg. Drop-Centre Rims".
With technology now, it doesn't make this variable so important to understand. Just hope that you don't run into an issue where a certain tire isn't compatible with your bead profile.