Compound stops

Compound stops have several ranks of pipes per stop. There are different types of compound stops, such as multiple fundament ranks, compared to multiple harmonics, or repetitive and non-repeating stops.

Multiple fundament stops are, for example, the "Bifara" with two 8' pipes (often as a double pipe with two labia) on one valve, or doubled strings (in southern German organ building, for example with Gabler or Freywiß), principals, or flutes (see the organ in Amsterdam). In a broader sense, these include undulating stops, either on a single stop or with separate stops like "Voce umana", a typical Italian undulating double principal. In other organ landscapes, undulating strings or flutes were popular. An off-tune rank can only give the undulating sound effect in conjunction with another of the same type and height (usually 8').

Since not every organist was familiar with how to combine the appropriate stops organ builders have often attached instructions, such as in the organ at Maihingen by Johann Martin Baumeister, who added in his inscription to the off-pitch Cythara 8' „mit der Flauten allein“ (with the Flaut alone). Another option, the technical connection of two ranks, was realised by Heinrich Trost in his organ in Waltershausen, where the reed stop Vox humana can only be played together with Hohlflöte 8' - alone or in other combinations the stop is without effect. The occurrence and the patterns of such multiple ranks are therefore very characteristic for individual organ builders or certain organ landscapes.

Mixture stops

The function of the mixture stops is reinforcement of harmonics. They have been used for centuries to synthesize certain timbres, for example to produce the sound character of reeds. A certain number of those synthetic stops contin repetitions in compass, others remain unchanged. These include the following registers.

Rauschpfeife 2f.: 2²/₃' + 2' (harmonics 3+4 to 8')

Sesquialtera 2f.: 2²/₃' + 1³/₅' (harmonics 3+5 to 8')

(Variant:)

Sesquialtera 3f.: 2²/₃' + 2' + 1³/₅' (harmonics 3-5 to 8')

Hörnl 2f.: 2' + 1³/₅' (harmonics 4+5 to 8')

Terzian 2f.: 1³/₅' + 1¹/₃' (harmonics 5+6 to 8')

Occasionally these registers repeat just once within the keyboard, for example Sesquialtera as follows: lowest octave first 1¹/₃'+⁴/₅', from to top 2²/₃'+1³/₅'.

Some of these stops are typical for certain regions: Rauschpfeife for example was a stop predominantly used in North German and Dutch organ building, the triple Sesquialtera is characteristic of some West German organ builders, while Hörnl is a peculiarity of South Germany, Austria and Switzerland.

Another special compound stop is most typical for French organs (but widespread far beyond France, following this example)

Cornet/Recit 8' 5f.: 8'+4'+2²/₃'+2'+1³/₅' (harmonics 1+2+3+4+5)

as a characteristic solo stop. Some built this stop only with four (without fixed 8') or three ranks (without 8' and 4') in order to allow the organist a certain selection of fundamentals. If the register was planned five-fold, i.e., as a complete combination, the 8' register was usually a stopped diapason or a chimney flute, the 4' register a principal. The Cornet 5f was usually not built over the entire keyboard, but only for the treble half (usually from c¹/middle c)*.

In contrast to the previously mentioned compound stops, mixtures repeat several times. These include the Scharf and Cymbal stops in addition to the mixture. Traditionally  they have in common several ranks of octaves and fifths above the fundamental (harmonics 2/4/8/16... and/or 3/6/12...). Certain organ builders also added series of thirds (naturals 5/10) to the mixture stops; such mixtures were later called "Scharf", although this register originally also had no thirds.

The older practice of organ making distinguished the three registers according to general altitude: the mixture had relatively deeper pipe ranks than the scharf, this in turn deeper than the cymbel (based on the respective fundamental). For example, if the mixture on C might have started on 2' (+1¹/₃'+1'...; harmonics 4/6/8 ff.), the scharf started on 1' (+²/₃'+¹/₂'...; harmonics 8/12/16 ff.) and the cymbal on ½' (+¹/₃'+¹/₄'; harmonics 16/24/32).

But these intervals over the fundamental could not be continued over the entire compass: Over C the highest note of a cymbal was c⁴ sounded as the highest note. Continued to an an assumed upper end of the keyboard of c³  this should result in a c⁸ with a pipe length of ¹/₆₄' (about 4 mm) - not only that an organ pipe like that is hardly possible to build and tune, its sound is already beyond human hearing in the ultrasound range.

The "repetition" of these registers therefore means that from a certain upper limit  (usually c⁵ or ¹/₈', the "plafond") the rank is continued at a lower pitch level, one octave or fifth rank lower

Example: Cymbal 3f: starting ½'+¹/₃'+¹/₄' over C 8'

In the repetition described here, the octaves and fifths within the third and fourth octaves would be added above each fundamental, regardless of the octave of the fundamental itself. At the upper end of the keyboard this ends in: 4'+2²/₃'+2' to the fundamental.

This kind of cymbal repetition between c/cs and f/fs (except in the lowest octave) would still be a bit too noticeable for the ear. But since the listeners should not notice the repetition - for example in scales - some organ builders have developed various methods to conceal the repetitions even more, for example by means of the individual ranks switching at different points and also more often. Another means is the doubling of pipes within the ranks (in fact the rule for a line-up with more than four or five pipe ranks).

The multiple and repetitive stops represent a very peculiar means of sound production for the organ, which has played a special role in the history of its sound aesthetics. Without such stops, an organ would lack some of its most important and characteristic sound components. Without the brilliance of the mixtures and cymbals, the sound of the "Plain jeu" is not complete.

* For designation of different octaves the traditional musical numbering practice is used here counting up and down from a "middle c"= c¹ by numerals above (up) and below (down; alternatives: dashes or doubling the tone letter depending on typeset or any technical limitations) as the centre of audible range, imprecisely labeled "Helmholtz notation" but in use in music and esp in organ making for more than 6 centuries, not the newly introduced so-called "scientific notation" recently used among physicists. Not only that music and history are no less scientific than physics, counting octaves starting from the lowest range of audible frequencies is musically senseless.