Does a "split curtain waistband" actually serve a purpose, or is it just one of those sartorial details? There's a reason. Think about sitting down... your body is constantly moving and changes shape. As you sit, if the pants were (as they often are) one continuous piece, then they'll gape at the back, which is why you see so many ass-cracks around. Even a belt only does so much to keep the pants pinned to the back. A split waistband solves that problem. You know how some jackets and zip cardigans bunch up when you sit down? and then some have a second bottom zipper you can pull up? Same idea.