I came across this email while cleaning out my Flute List e-mail folder. It was written by Mary Byrne, Ph.D.
Faculty, Victoria Conservatory of Music
Victoria, BC CANADA. A great little insight on double tounging and how to teach it. If your not a member of the Flute list, this is one reason why you should be!

Subject:
Re: Double tounging
Date:
Tue, 30 May 2000 16:44:08 -0700
From:
Michael/Mary Byrne
To:
FLUTE@LISTSERV.SYR.EDU

Dear List,

This question has been answered in many different directions…all of them
interesting!

I teach double tonguing at the point that a student simply needs a speed of
articulation that her/his single tongue cannot provide. Before I teach
double tonguing, I want to know that the student is in possession of a solid
single tongue and a strong tone with good use of air. These factors can be
satisfied at almost any time in the life of a flute student.

I have encountered useful books on the subject of double tonguing–top of
the list being Trevor Wye’s Practice Book. There are also some good–but
small–exercises in the Rubank Advanced Method Books. However I use these,
and other sources, as tools rather than instruction manuals.

When instructing students in double tongue, I first remind the student what
makes up a good single tongue, i.e. a clean attack that does not block the
airflow more than is absolutely necessary to create the articulation. To
this end I will work for a few weeks on tongue-less attacks to make sure
that the tone is not reliant on the tongue for its production. In
conjunction with this we employ the “T” articulation stroke only to clarify
and clean-up the start of notes. This can be practiced on scales, simple
steady-rhythm etudes, or basic technical studies.

Once this is set we repeat the process, this time employing the “K” stroke
instead of “T”–this is to strenghten the “K.” I stress a forward placement
of the “K” within the mouth. Pronouncing the words “Kitty” and “Kite” and
“Cut” the student will readilt find that the “K” hits in three different
places. I like to use the forward stroke of “Kitty” as this minimizes the
motion required of the tongue in double tongue and seems to be less
disruptive to the air flow. Again this can be practiced on scales, simple
steady-rhythm etudes, or basic technical studies.
I may also ask students to replay familiar repertoire using “K” instead of
“T” as the basic articulation stroke.

In the teaching process, I am careful to compare the sound of the “K” to the
sound of the “T.” I am also careful to stress proper and abundant air use
with the tongue as this is a very important component for “masking the
click” of the “K” stroke.

When the “K” is strong and virtually indistinguishable from the “T,” we move
on to “proper double tongue” studies, exercises and etudes working the
alteration of “T” and “K.” This may not be an easy transition, but the
ground work is well-laid and many students do come to it very quickly. Of
course the two problems that most often come up are speed and coordination
with the fingers. Coordination with the fingers will come with time and
attention. Speed will as well, but one exercise I have recently found
useful in this regard is to have students say “Kitty” over and over and then
to suddenly blow alot of air through the mouth while continuing to say the
words. Most people will find that the tongue suddenly flies forward and
fast as free as can be. Then it’s just a matter of coordinating that with
the flute. I leave that to the individual creativities of the teachers out
there.

As to the other topics around this subject:

I do not agree that double-tongue is an extended technique of the flute. It
may not be necessary, but it is apart of the technique of the
instrument…as is vibrato, and dare I say multiphonics, etc….

I do not agree that double tongue is a substitute for a slow tongue. I
consider it merely an extention of a proper single tongue. LOTS of
different consonants are useful for different musical objectives…T, K, D,
G, L, N, P, H…

I do not agree that double tongue prevents a performer from developing a
quick single tongue. I have, by all accounts, an extremely fast double
tongue–I have actually gotten it to mimic flutter tongue in certain
circumstances. However, my single tongue–once quite slow– has worked up
to quite a perky tempo (sixteenths at 132-ish). I find now that there are
many times I flip back and forth between single and double tongue within a
single bravura passage according to the demands of the music. It’s a little
like going between two languages! I also doubt that my single tongue would
have improved as it has were it not for the work with my double tongue.

Well, that’s it. Hope some of this helps!

Mary Byrne, Ph.D.
Faculty, Victoria Conservatory of Music
Victoria, BC CANADA