(this may be playing the devil's advocate, but here it goes) Two hundred years ago I would ask do the the "fit" or "hang?" Since then our use of "trousers" and "pants" has changed. Here on the forum, we are continually dancing around this issue as we use historical terms both in a modern way and the original sense. So it may be more revealing not to ask how many seams, but do they pull up at the knee when astride a horse? If they do, they are trousers. If they accomodate in the seat, they are trews. Of course, modern usage I expect to be totally different. Thank you for openning this issue, maybe someone can answer in the modern sense.