A variety of Diapason with a horn-like tone, resulting from the pipes being slotted. While the organ builder Walker & Sons of London, England apparently favored this stop, as did Wurlitzer somewhat later, the tone of the Horn Diapason is rather maligned in the literature. Audsley describes it as having �a horny and somewhat stringy quality which fails to satisfy the sensitive musical ear�. Bonavia-Hunt says �the quality of tone imparted by the slot to large or moderately-scaled diapason pipes is objectionable on account of its hybrid nasality�. Wedgwood calls it �horny and sometimes somewhat �sugary� and cloying�, and says that it �rapidly becomes wearisome�. Not surprisingly, the Horn Diapason found a home in the theatre organ. Strony writes: A very useful top that is important in softer and mezzo forte accompaniment registrations. It is also important in building mezzo forte registrations on the Great. Unfortunately, it is missing from far too many instruments. The Horn Diapason is one of those �in between� stops that is bright; yet it has a good deal of fundamental. Because of this, it is an excellent binder for the flutes, strings, light reeds, and larger diapasons. Wurlitzer included the Horn Diapason in many instruments between 13 and 17 ranks, and in most of their larger organs. The 16' pitch is sometimes called Diaphonic Horn; the 4' pitch is sometimes called Octave Horn. Irwin claims that it is not cloying, calling it �brass-like but not brilliant�. Maclean likens it to the French Montre in tone, and states that it �possesses in a very high degree an uncanny faculty of blending with almost any other kind of pipe tone�. According to Bonavia-Hunt, the slots, which favor the 5th, 6th and 7th partials, are cut one pipe diameter away from the top of the pipe, and the widths of the slots may vary from 1/3 to 1/5 of the pipe's diameter. Grove dates it from the late 19th century. In The Art of Organ-Building Audsley describes the Horn Diapason as being of inverted-conical construction, which can have tonal effects similar to slotting. However, in his stop dictionary, published later, he says nothing about Horn Diapasons being constructed in this manner, nor does any other source.
Osiris contains two dozen examples, all at 8' except when occasionally unified to 16' and 4'. Nearly a third are by Wurlitzer. The earliest examples are given below.