Bee space, and why frames must fit

Langstroth's whole invention was one measurement, six to nine millimetres, held everywhere two surfaces in the hive face each other: frame to frame, frame to wall, box to box. Every dimension of a National frame or a Langstroth frame is cut to keep that gap through the whole stack.

That is also why the two systems do not mix. A National frame's lugs do not sit on a Langstroth box's rails; a Langstroth frame is the wrong depth for a National super.

Put the wrong frame in the wrong box and the gap goes outside that six-to-nine-millimetre band somewhere in the hive, and the bees do what bees have always done with a bad gap: seal it or fill it, in wax, on their own schedule. Choosing a hive type means choosing every box and frame you will ever add to it.