The separate stop-chest made it possible to operate a stop from more than one keyboard, or at more than one octave, a process which is called duplex, multiple or unit stop control. Noted builders are applying the idea in great variety. The principle is not new. It was brought out in Belgium by L. Dryvers, and described by H. V. Couwenbergh in 1887. One of his schemes comprised an organ of six units, from which a three-manual organ of forty-six registers was formed. For instance, a Bourdon stop of 104 pipes yielded ten registers, of the following variety of nomenclature--_Bourdon_, _Sous-Basse_, _Flûte Bouchée_, _Flûte Douce_, _Flûte Champêtre_. The ingenious prophet, however, added to this scheme a _Récit_ organ of eleven absolutely separate solo stops, built on the _système ordinaire_, and expressive, thereby showing a commendable sense of the weakness of his own system! All modern organs employ the principle of duplex mechanism to some extent, and, legitimately used, it is of enormous value. The example given above is the _reductio ad absurdam_ of the idea, and also indicates the deceptive habit of renaming the stops thus derived. The success of the modern organ has depended in large measure on the use of really effective swell chambers. Not only are they effective, but the proportion of stops that are enclosed has been greatly increased. The organ has thereby been liberated from its old lack of flexibility. We even find two expressive divisions playable from one manual. An interesting adaptation of this idea is the grouping of all the stops of each tone family in separate swell chambers. This has been done on some large concert organs, as well as on those of the unit type. Mention must here be made of the conspicuous service rendered by Robert Hope-Jones both in his insistence on effective expression, with the stops arranged in ‘families’ of tone, and in his advocacy of the unit organ. However, he was often obliged to modify his own theories in practice. He was the first to leather the lips of Diapason pipes.