In Praise of Back Dimples

If I had to choose a (non-gender specific) Favourite Body Part, it would have to be those two little dimples where the top of the bum and bottom of the back meet. I've always liked them but I think the prevalence of low-riding pants for both men and women has brought it to the fore. As far as I know, most people who have an ass and a back also have these dimples. I could be wrong though.

Does it have a name? Why do they even happen? I hereby christen thee "back dimples".

I wanted a photo for this. I suppose this means I need a digital camera.