Organs have no fixed number of stops; some have sixty or more, and others much fewer. A stop is a collection of pipes similar in tone and quality, running through the whole or part of an organ. They may be divided into mouth-pipes and reed-pipes, according to structure, or into (1) metallic, (2) reed, (3) wood, (4) mixture or compound stops, according to material. The following are the chief:
(1) Metallic. Principal (so called because it is the first stop tuned, and is the standard by which the whole organ is regulated), the open diapason, dulciana, the 12th, 15th, tierce or 17th, larigot or 19th, 22nd, 26th, 29th, 33rd, etc. (being respectively 12, 15, 17, etc., notes above the open diapason).
(2) Reed (metal reed pipes). Bassoon, cremona, hautboy or oboe, trumpet, vox-humana (all in unison with the open diapason), clarion (an octave above the diapason and in unison with principal).
(3) Wood. Stopt diapason, double diapason, and most of the flutes.
(4) Compound or mixture. Flute (in unison with the principal), cornet, mixture or furniture, sesquialtera, cymbel, and cornet.
Grand organs have, in addition to the above, from two to two and a half octaves of pedals. Stops, strictly speaking, are three-fold, called the foundation stop, the mutation stop, and the mixture stop. The foundation stop is one whose tone agrees with the normal pitch of the digital struck, or some octave of it.
The mutation stops produce a tone that is neither the normal pitch nor yet an octave of the digital struck. The mixture stop needs no explanation.
Among varieties of organ-stops may be mentioned the complete stop, which has one pipe or reed to a note. The compound stop, which has more than one pipe or reed to a note. The flue stop, composed of flue-pipes. The incomplete (or imperfect) stop, which has less than the full number of pipes. The manual stop, corresponding to the manual keyboard. The open stop, which has the pipes open at the upper end. The pedal stop, as distinguished from the “manual” stop. The solo stop, the string stop, etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894