Technically it should be a waist measurement.. but depending on the cut the "waist" can be in different places! I find that TS's are cut very curvy: smaller in the waist, roomier in the hips That's exactly opposite of how I'm built so either I can't zip the waist or I look like I'm carrying a load in my diaper. Ariats are cut more straight through the hip so they fit me MUCH better. I'm a 28 in Ariats and sort of a 30 in TS's. I'm a 28 in Equine Couture and Ovation as well.
PS- if you're a 28-30 you are no cow!! Unless you're calling me fat... in which case, MOO!