Frame size?

scotland Sorry to clarify, do you guys mean concave/convex relative to the ground? I think you need a point of reference to describe something as concave/convex. my back sometimes feels concave sometimes convex. Buffalo Bill described a "natural S shape", does that mean one part of the back concave and one part convex? If so which part is which. Thankx, I'm trying to sort out bike fit and posture too.

I think he means concave at the lower back and convex at the upper back in order to back the 's' so that you can lean forward but so that you are not hunched over