A formal trouser is not simply a suit trouser worn separately. It is a garment with its own logic: cut to sit at the natural waist, to drape cleanly through the leg, and to break at the hem in a way that is neither too short nor too long. These are suit trousers and tuxedo trousers from the same houses that make the jackets they accompany, in the same cloths, to the same standard. The trouser that makes the jacket above it look like it was always meant to be there.