A short octave is found on many early keyboards. The advantage of a short octave system is that it sacrifices little-used notes for economy of size (keyboard and instrument case width) and materials (added strings, pipes). The most usual form is the C short octave, where the lowest key (apparent E) plays low C. The natural keys F, G, A and B all play their proper pitches, but the F-sharp and G-sharp keys play low D and E respectively, leaving the notes C-sharp, E-flat, F-sharp and G-sharp unrepresented in this octave.
Meantone tunings, use slightly narrow fifths, but pure major thirds, corresponding to the numerical ratio 5:4. Meantone tunings became prevalent in the early Renaissance with the emerging emphasis on triadic harmony. It allowed more modulations than the Pythagorean tuning characteristic of chant and early polyphony in the Middle Ages. In the most frequent and characteristic quarter-comma meantone, one tunes E pure to C, then tunes the fifths between C and G, G and D, D and A, and A to E so that they beat equally, thus dividing the syntonic comma in four equal parts. In this system, a string or pipe tuned to E flat (a pure third from G) can serve as D sharp (a perfect third from B) only with difficulty—the sour interval or wolf tone is very noticeable. Some organs were built with doubled sharps--the front and back of the black key would address different pipes, say D-sharp and E-flat. Mexican organs do not employ doubled sharps--the chords of C minor or E-flat major ring true, while the B major chord (dominant of E minor) sounds decidedly sour, and the fifth between G sharp and E-flat is even worse.
Meantone tuning was prevalent on keyboards during the 17th century, and is suitable to most keyboard music written during the period, including that employing liturgical chant or chorale melodies; it is especially suitable to Iberian music of the Renaissance and baroque periods. By the late 17th century, some theorists were advocating circulating temperaments with both fifths and thirds tempered, allowing a much greater range of keys. Meantone, however, remained in use on organs in some parts of Europe well into the 19th century.
Most of the historic organs of Oaxaca employ a mechanism typical of European organs from 1500 to 1800, specifically a variety common in Spain and Portugal in the 17th century. The pipes stand in rows upon a slider chest, a type of windchest divided into vertical channels corresponding to each key. The sliders or registers are perforated wooden battens that slide laterally at the top of the chest; when the slider is drawn (or in some cases pushed), the holes in the slider line up with the pipe holes for a given stop. The registers either protrude beyond the side of the case, or, more typically, are controlled by stop-knobs on either side of the key desk.
The wind-trunk ends in a box below the windchest, where a pallet (valve) allows wind into each channel, and thus into each every pipe for a given key whose register is open. Thus, there is one pallet and channel for each key, and one slider for each stop, resulting in a minimum of moving parts.
In Spanish and Mexican organs employing divided stops, the windchest and registers are divided: the stops on the right side of the case or keyboard affect only the channels from middle c'-sharp upward, while those on the left side affect only the bass from middle c' downward. Conduits supply wind to the facade pipes, and to any horizontal reeds.
In a suspended tracker action, each key is pivoted at the rear. When the key is pressed, it pulls upon a flexible wooden strip (tracker) ultimately connected to the pallet that admits wind to its respective pipes. A well-balanced suspended action combined with a rollerboard offers a light, sensitive touch and quick response. All but the smallest Oaxacan organs employ a suspended tracker action.
Oaxacan table organs organs do not use a tracker action but may employ other linkages between keys and pallets, such as a pin action where pins or stickers mounted below the keys open the pallets. At Zautla, for example, according to James Wyly (in personal communication), the windchest and palletbox sit directly on the table that supports the instrument, somewhat below the level of the keyboard. The keys are very short, pivoted just behind the nameboard, so the pins cannot open the pallets directly; their downward motion must be transferred toward the rear of the case through a system of intervening backfalls (levers). The action is consequently stiff and uneven.
A rollerboard is the control center at the heart of most mechanical key actions. It is a plank mounted between the keys and the palletbox below the windchest. Attached to this plank are rods or dowels in horizontal position, free to rotate on their own axis. To each rod are attached two arms or hooks, one directly above the key, and one directly below the pallet. When a key is pressed, the tracker pulls the hook downward, rotating the roller; the other hook consequently moves downward and pulls the upper tracker, which opens the pallet.
A rollerboard enables windchests and cases to be built wider than the keyboard, so that the larger pipes can fit on the chest. It also allows the channels and pipes to be placed in something other than a strictly chromatic arrangement. On most European organs, the pipes alternate chromatically between the two sides of the case—there is a “C side” and a “C-sharp side.” On most Iberian and Mexican organs, however, all the channels corresponding to the bass keys are on the left side, the treble to the right. This allows the rollers to be shorter than on other organs, since they do not need to extend beyond the middle of the rollerboard. As a result, the rollerboard can be relatively narrow, with two rollers abutting one another end to end, as on the organ at Jalatlaco.
An additional refinement was noted at Jalatlaco. The hooks above the keys are relatively short, while those leading to the pallets are relatively long. This means that a relatively shallow motion of the key can produce a relatively large motion of the pallet, admitting abundant wind to the channel and pipes instantly, without effort or sluggishness, even in quick musical passages.
On smaller organs with suspended tracker action, the windchest may be narrow enough so that a rollerboard may not be necessary. This is the case at Tlacochahuaya, where the key trackers are splayed in a fan-shaped arrangement, and operate the pallets directly.