Traditionally the trouser seat is hand stitched utilizing a backstitch. Backstitch has the advantage of being a stronger as well as a more flexible seam than a plain machine seam. Backstitch looks from the upper side like an ordinary plain seam, from the underside, as the stitches overlap, it is like double stitching.
Ah. Thanks for the info. Okay, I figured out what the Post meant by "double seam"; it appears to be the same principle as a single needle shirt seam: the entire length of the seam is sewn twice. Except in the case of the trouser back seam, it is sewn once on each side, rather than twice on the same side. I looked at all my Savile Row suits: none of them appear to have a hand-sewn back seam. Neither do my two suits from Naples, which otherwise have absurd amounts of handwork. Maybe I am unlucky.