Originally Posted by a tailor
both legs should hang straight. this is commonly caused by one of three things. 1.the twisted leg was not cut on the straightgrain of the denim, a cutting room mistake. 2. the denim was twisted when spread on the cutting table. 3. when the seam is sewn one side of the seam is allowed to slip, allowing the seam to twisttward the short side of the seam. this is the most common cause. are the offending jeans are all of the same make? if this does not happen with all jeans you own then it is a flaw.
basically, what cheap mofo said.
unsanforized vintage repro jeans don't follow the same rules, mr. tailor. vintage reproduction brands mimick the "leg twist" that happens when denim shrinks. because of the directional weave, a right hand twill will pull to the right, bringing the outer seam on the left leg towards the center. this is done on purpose to reproduce the look of vintage jeans. now, on unsanforized jeans like the present-day STF, this is corrected so leg twist will not happen.
i think the person who started thread is talking about something else. maybe the way he folds his jeans is creating a crease?