I'm guessing that you're meant to sew the button into the trousers yourself (or have a tailor do it) for this exact reason. The extra flap has to go in that specific place on the shirt, but you could sew the button into the trousers at any height. Also, I think it is supposed to be a shirt button, given the size of the holes. I do wonder, though, whether the BB site got it right and that hole is meant to secure the shirt to the trousers or the waistcoat. You'll notice that the shirt referenced has three stud holes, and that the flap with the buttonholes feeds from where a the fourth buttonhole would usually be. Now, look at BB's white tie waistcoat: http://www.brooksbrothers.com/White..._______,default,pd.html?omnpp=recently viewed That fourth button hole position is directly behind the waistcoat, not inside the trousers. On black tie, I was always under the impression that the buttonhole inside the cummerbund was meant to fasten to one's shirt and now I'm just wondering if they've done the opposite for white tie. Having the shirt attach to the waistcoat actually makes more sense; the other would be very uncomfortable for sitting/standing and this way any untucking that happens happens behind the waistcoat but the front of the outfit appears perfectly in line all night.