Open an old door and look at its edge. If there's a rectangular metal case set into the wood, with the latch and bolt emerging from it, that's a mortise lock, and it shows a level of mechanical craft the modern equivalent gave up.
The mortise lock sits inside a pocket, the mortise, cut into the door's edge. That's the key difference from the modern bored lock, which drops into two round holes drilled through the door face. The mortise approach is more work to install but produces a stronger, more integrated lock, with the whole mechanism housed in a solid case rather than clamped through a couple of holes. Inside that case is genuine mechanism: levers, springs, the latch and the deadbolt working off separate actions, assembled to run smoothly for a century.
That durability is the practical appeal. A mortise lock cleaned and adjusted usually works as well as it did new, because it was built to be maintained rather than discarded. The matching challenge is real, though. Mortise locks came in many sizes and configurations, and the trim, the knobs, escutcheons, and backplates, has to suit both the lock and the door. Getting a complete, correct set is the work, and it's what we describe plainly in each listing: what the set includes, and its condition.