A 38JJ is a 38 band with a JJ cup — a specialist, very deep cup. The band does most of the work (around 80% of your support), so if it rides up at the back or the straps dig in, it's the band to fix, not you.
We cut 38JJ for fuller busts, part of our full 38D–38K range. Not sure 38JJ is your size? Book a personalized fitting and we'll get it sorted.