I've been looking around for a pair of slim chinos and I think every pair I've seen (if not all of them) have that line that runs from the back pockets to the waistband.
If you don't know what I'm talking about you can see it here, it lines up with the side of the tag on the waistband (put the mouse pointer over the picture to zoom):
Is this something that is for some reason necessary for the cut of slim pants, like darts on a jacket? I don't see why it would be. Is this just an ornamentation? I find it unsightly but it seems impossible to find a pair of pants without that.