The 19th century simple system flute is an amazing device. In addition to the possibilities provided by the keys, one may still use the techniques of forked fingerings and partial covering of holes. A huge palette of pitches and timbres are available. I find that medium-holed flutes have the most flexibility in this regard, though harmonics and partial covering work very well on large-holed flutes. What goes on inside the bore with some of these fingerings must be incredibly complex.
(The instrument has been used, and could well be further exploited, by contemporary composers and players who wish to experiment with timbre and pitch. It would be relatively easy to create quarter-tone scales or third-tone scales, etc.; but a problem is that each flute would need somewhat different fingerings.)
In the late 19th century, ideas and flutes were changing and players are advised (by, e.g., M. Schwedler) to limit the number of fingerings they used. It is always advisable to learn selected fingerings well rather than always trying to rely on alternate fingerings to solve technical problems. It is perfectly possible to play 19th century music without alternate fingerings. But there is no doubt that the virtuoso players of the early and mid-19th century used and reveled in a multitude of fingerings. This can be seen by studying the methods and tutors they wrote. Click here for www.flutehistory.com's list of flute methods. A number of these are available in modern facsimiles. The methods allow one to understand a particular player's "system" of fingering (on the type of flute they used) in a way that a comprehensive fingering chart cannot.
The various fingerings were used to simplify finger motions and otherwise increase smoothness of some passages (e.g. by avoiding register breaks), to ease intonation difficulties or change intonation for expressive purposes, to control dynamics, and to produce different timbres. A number of third-harmonic fingerings (unassisted, assisted, or sharpened by extra venting) are particularly useful.
The following is taken from Sections 781–2 of Rockstro's Treatise on the Flute of 1889. (There is no Rule III. I don't know why.)
I am aware that some persons are opposed, in theory, to any departure from the ordinary fingering of a flute, but as all the best flute-players, from the time of Hotteterre, have been glad to avail themselves of all the resources of their instruments, and as not one of the opponents of the use of extra fingerings has been able to carry out his ideas with success, it must be accepted as a fact that occasional changes are indispensable.
In the selection of extra fingerings the following general rules should be observed:I. Never use different fingerings, which cause an alteration of pitch, for similar notes that are near each other.
II. Do not select a sharp fingering for one note, and a flat fingering for another which is adjacent to it, but on the contrary, choose a set of sharp, or a set of flat notes, so that all may be corrected together by a slight change in the method of blowing.
IV. For slow music, select the fingerings which give the best notes, no matter how difficult they may be. Indifferent notes may occasionally be tolerated in very rapid passages, for the sake of smoothness and facility.
V. Avoid all unnecessary changes. The chief object of extra fingerings being the preservation of smoothness, they will be needed comparatively seldom in staccato passages, for which many of them would indeed be wholly unfitted.
VI. Use the harmonics (unassisted by vent-holes) of the second and third series, only as a last resource. ...
VII. Any finger-holes may be kept closed during the performance of rapid passages, provided no appreciable injury to intonation or tone be caused by such closing.
A sensitive note (note sensible, in French) is a note that is raised in pitch, by perhaps 1/3 of a semitone, in order to narrow the interval with the note a written semitone above. One does not see this concept explicitly named in the 19th century German sources that I know, but fingerings that give sensitive notes are included and often used in the same situations.
The change in fingerings from the sensitive note to the one above is supposed to be easy; one wants a smooth transition. (This is important even if the interval is not narrowed.) An excellent example of a sensitive note is the c''# given by closing the low C# key while fingering d'' as -23456. This is sharper than the normal c''# with all holes open, and can be alternated with d'' easily.
Here are some typical instances in which sensitive notes may be used:
Here are some figures extracted from a Walckiers duet where sensitive fingerings may be used for the notes that I have marked "S".
The first figure provides a chance to use 123-56 (no key) for d'''. Try fingering the first and third notes of the second figure as 12-456k and use 12-456 for the a'' between them. For the b'' in the third figure, use 1--456k. Try 123-56 or 123-56k for the f''# in the fourth figure; but it may not work so well on every flute. I find that the figures sound better with these irregular fingerings, not that I remember to, or bother to, use them all the time. (Also, on the third figure, try 1-34-6k, 1-3456k, 1-34-6k, -23---k, and then open the G# key for the e'''b. The b'' is not sensitive, but the figure is smooth.)
Rockstro (1889) remarks that notes may be advantageously lowered when between two notes a written semitone below. Rockstro was an opinionated advocate of the modern flute but still used these 19th century inflections. This is from Section 767 on "Altered Semitones".
...there are certain cases in which a slight and temporary departure from [equal temperament] may be permitted. These cases result in the lessening of a semitone, either by raising the lower, or depressing the upper, of the two notes which form the boundaries of the interval, thus causing some approach to the semitone of Pythagoras .... This reduction of the semitone, if not carried to an extreme, is generally as agreeable to the ear as the slightest enlargement is offensive, but on no account must a note be raised or lowered when it forms an integral part of the accompanying harmony, or when it is near to a similar note which not so altered from its normal pitch. The raised lower note the semitone is known in France, where it is much used, as the "note sensible," or sensitive note. I know of no technical name for the depressed upper note of the interval.
In the following example, Rockstro uses "S" to indicate a note to be sharpened and "F" a note to be flattened.
Sensitive notes must never be so sharp that they sound out of tune. Do not hesitate to attempt to correct the intonation of sensitive fingerings that may go too far in narrowing the interval. The smooth effect, and color, is more important than the exact interval. In many instances, I like a change of color in a sensitive note: a softer, more veiled or sweet sound than the upper neighbor. It is important that the listener feel—if the listener is explicitly aware at all—that this play with intonation is being done purposefully, to tease or flatter.