Benedict Pentenrieder (1809–1849) worked in Munich; his shop operating from 1836 to 1854. The seven-key flute shown at the top of the following photo is similar in profile, materials, and key layout/shape to some of the less expensive instruments by other Munich makers (including Theobald Boehm and Rudolph Greve) from the same period, c.1830–40.
The introduction of Boehm's 1832 model flute presumably influenced Pentenrieder, though he denied any connection to his work. In fact, Pentenrieder's innovations were not related to Boehm's scientific placement of finger holes and Boehm's fingering system at all! Instead, Pentenrieder used crescent keys, and later ring keys, axles, and needle springs (that is, the mechanisms that Boehm made practical and popular) to implement his ideas on enhancing the simple system flute. A example of his mature construction is the second flute in the photo above, the middle section of which is shown below.
The playing qualities of both Pentenrieder flutes are similar to a simple system flute flute by Boehm and Greve that I have tried, with easy and bright high notes. I was told by the dealer from whom I acquired the instrument, that his source claimed that the c.1845 Pentenrieder flute was used by an ancestor in a Strasbourg orchestra in the 1870s. Whether this is accurate or not, the flute's bright sound would have worked well in that context. The shape of the keys and levers reminds one strongly of the keys and levers on the conical Boehm models of Boehm and Greve.
Penterieder produced flutes with from one to four articulated keys. He claimed to be the first to use this idea on the flute and clarinet. Here "articulated" means that the key assembly is in two rigid pieces. One piece (orange, below) consists of the key cup/pad and a ring key, and a weak spring tending to hold the key pad up. The other part (green, below) contains the key touch, a short lever resting on the key cup of the first part, and a strong spring which overpowers the weak spring and forces the key cup down.
The most common and useful of these keys is the "articulated G# key". It may be found on oboes, saxophones, and some clarinets today. The key acts like a normal closed-standing G# key in that depressing the touch causes the cup over the G# hole to rise; the ring key over finger hole 4 also rises. When RH1 presses on the ring key, the G# hole is closed even though LH4 may remain on the G# key touch. This simplifies many passages and figures very significantly. In particular, alternating G# and F# is now a one finger movement, and the trill is perfect. Using G# with any of the notes F#, F natural, E, D#, or D below no longer requires the release of the G# key, if the player so chooses, when the right hand fingers close their holes. The Ab/G trill can now be done with RH1 on the ring only, rather than with LH4.
The construction of the articulated keys, with their opposing springs, is similar to that of the Dorus G# key for the Boehm flute, except that in the case of the latter, the relevant key pad (for the G# hole) is open-standing at rest, closed by the ring key for LH3, and opened again with the lever.
The ring keys do interfere with some effects and alternate fingerings of the simple system flute. For example, the sensitive fingering of g''#, with fingers LH1, LH2, RH1, is no longer available. Because of the changes in capabilities, whether judged positive and negative, I no longer consider the Pentenrieder flutes to be simple system flutes.
Here is a summary of the action of the four articulated keys.
I have found many uses for all these keys! For example, the second key above makes alternating Eb and F a one-finger movement. Yet Pentenrieder's ideas seem not to have been taken up by other makers at the time and Pentenrieder died unappreciated. It is likely, though, that his articulated keys influenced some makers at the end of the 19th century and early 20th century.
The articulated G# was added to numereous early 20th century flutes. The Modell 1895 Schwedler flute shown below has the ring keys for finger holes 1, 2, and 5 introduced by Schwedler (for his Cis-brille and Fis-brille, the latter of which is an articluated key), but also a ring key for finger hole 4 that is part of an articulated G# key (which was not part of Schwedler's design). The G# cup has a short lever for the Ab/G trill with RH1.
M. Pupo Pupeschi (1859–1932) was a Florence wind instrument maker and inventor who in an 1892 patent proposed improvements for the clarinet and oboe. His G# key assembly was articulated and in addition involved a clutch to the LH4 keys used for the very low notes on those instruments. Pupeschi asserted that his ideas could be applied to other instruments. In his Die Flöte of 1905, Paul Wetzger illustrates a "Pupeschi system" flute as follows.
The four ring keys are on the same finger holes (2, 3, 4, and 6) as the rings on Pentenreider's flute. However, the details of the construction and function differ.
In an advertisement near the end of Wetzger's booklet, V. Kohlert's Söhne claim that they are the sole makers of the patent Pupeschi system flute in Austria and Germany, etc., and they include the following illustration of a flute with two ring keys for finger holes 4 and 6.
Here is an actual instrument from the Kohlert firm. It is a Schwedler-Kruspe Modell 1885 flute with the addition of a special mechanism for the right hand.
Here are closer views of the center joint.
The instrument has an ordinary (not articulated) G# key, perhaps left for the convenience of players used to it, and perhaps for cases when it is desired to have the G# hole open but the F# key depressed (e.g., in some fingerings for a''' and b'''b). But the long F key acts as an articulated G# key, and more.
Depressing the long F key raises both a second hole for G# and a second hole for F. So with holes 1,2,3 closed and the others open, the long F key sounds G#. While the long F key remains depressed, the ring key for hole 4 closes the supplementary G# hole, and the ring key for hole 6 closes the supplementary F hole. Thus, if the long F key is used for sounding G# (rather than the normal G# key), then, without releasing the long F key, adding RH1 gives F#, adding RH2 gives F, and then adding RH3 gives D (or Eb if the Eb key is depressed). The rings have the same function as the ring keys for 4 and 6 on the Pentenrieder flute, but they are both attached to the long F key, rather to the G# and F keys.
This is very clever. As Wetzger points out, this device solves the problems of the simple system flute with the notoriously awkward sequences D-F-Ab and Eb-F-Ab. The figure D-F-Ab-F-D-F-Ab-F in the first two octaves can be repeated endlessly at extremely fast tempos without any use of keys (other than the automatic ring keys) after the initial depression of the long F key.
Another appearance of ring keys for some type of articulated keys is in a Rampone catalog of circa 1923. The firm advertises a "Tulou system flute", which is available with no, two, or four ring keys. (The original Tulou flûte perfectioneé had no ring keys.)
Rampone says their Flauto sistema Tulòu with four ring keys facilitates many trills and passages while preserving the fingering of the Flauto sistema Ziegler, i.e. the simple system fingering. They say the four ring keys are "articolato" and for Bb, G#, F#. I do not completely understand this and I cannot tell if these are Pentenreider's keys or Pupeschi's keys, though they are perhaps more likely to be the latter's.