All a Flutter

Ever heard a really bad imitation Scottish accent? With lots of RRR rolling? Ever done one? Then you can flutter tongue.
Flutter tonguing isn't used that often but it turns up more and more as time goes on. It can be found in Jazz, Classical and even Folk music. The rolling of the tongue creates a buzzy, surreal sound that mixes with the clear flute sound in interesting ways. The sound can't be imitated very well by any other technique. And its fun.
(A personal plea; Keep the jokes about this topic clean please! Trust me, we've all heard the dirty jokes before and they get old fast.)

There are two ways to flutter tongue and both relate to rolling Rs. Really, both are extremely fast and repeated tongue strikes that don't stop the note or air stream. This is not a muscle controlled action; the tongue is close enough to the roof of the mouth that the air stream causes it to vibrate like a reed on a clarinet and this creates the buzzing or rolled R sound.
The preferred technique (for flute) uses the tip of the tongue. Put the tongue behind your front teeth and let it "bounce" off the roof of your mouth as you exhale, like a rolled R without vocalizing. (This is the same spot you place your tongue for a regular single tongued note-the "ta" or "da" tongue.) It is sort of like turning your tongue into a mini jack-hammer. The trick is learning to maintain your embouchure while doing this.
The other method uses the back of the tongue or the uvula R roll, the way Rs are rolled in French. Its not that different from the other one, just further back in the mouth. This is the spot you use for the "ka" or "ga" sound when double tonguing. Just faster. For the record, flutter tonguing is "easiest" for flute and brass instruments but the reed instruments can flutter too (yes, even oboes though that may count as cruel and unusual punishment.) It is however much easier for reed players to use the second method of flutter tonguing!
If you can't do one, try the other. The first method tends to be more obvious sounding and the second one has a softer buzz but either one works. If you can do both, you end up with a choice of sounds for flutter tonguing. Always a plus.
Once you get the hang of it, its actually kind of entertaining to do although having to switch it on and off at just the right moment can take some practice. Often, flutter tonguing is used on one or two notes though they may be long held notes that make your mouth feel odd after awhile. Sometimes the flutter is meant to last over several notes or used with faster passages. With luck, it will be clearly marked when to start and stop.

So how is this marked in music? Unfortunately, its not very consistent. Part of the reason is that, just like all terms in music, different languages use different words. Flatterzunge, frullato, coupe de lange roule, tremolo dental, vibrata linguale are just some of the different names used for flutter tonguing.
Most flute players agree that it SHOULD be marked with a fl, flt, flz or fltz and sometimes frull. or flutter above the note followed by a squiggly line (a trill mark) to show how long to flutter.

This is clear, noticeable and informative so of course many composers don't use it.
The next most recognized marking is to write fl or flutter above the note with diagonal lines slashing the stem of the note (a string tremolo mark.) This works but it can be confused with subdivision markings especially since composers often leave out the the fl and only use the slashes or write flutter once and never again even when the fluttered notes are mixed with regular notes. I recently played a piece that used this mark for flutter tonguing right after using subdivision marks for actual subdivisions and then proceeded to switch back and forth from subdivisions to flutter tonguing for two lines. It took me 4 times through the piece to figure out what was happening where.
Some composers simply write FL or flutter above the note with no other marking and leave you to guess how long you have to keep your tongue buzzing. Quite frustrating.
And some composers switch between all these marks just to make sure the flute section is paying attention. These are the composers I want to track down and send to remedial composition classes!
Finally, other composers make up their own marking I guess because they couldn't be bothered to look it up. As long as they explain their invented mark clearly and they use the same mark for the whole piece, I don't mind this. Consistency makes up for a lot!