Double tonguing is a technique used by players of brass instruments when merely using the tip of your tongue to separate the notes isn't going to be fast enough- say, for uptempo sixteenth notes, most thirty-second notes, slow tempo sixty-fourth notes, and so on. Compared to flutter tonguing, double tonguing is a much more precise way of enunciating individual notes.
It's known as such because it uses more of your tongue in order to work- whereas normal tonguing produces a "tuh" sound by using the tip of the tongue to interrupt the air flow, double tonguing adds a motion with the back part of the tongue, producing a "kuh" sound. In combination, it goes "tuh-kuh-tuh-kuh" for as long as you need to make lots of short notes.
If the "kuh" part of the double-tongue isn't done completely so that the air flow is completely shut off, the entire thing sounds somewhat wimpy. Of course, like most techniques involved in playing a musical instrument, practice makes for improvement- and if you can tongue really, really fast (and by that I mean really fast), you may not need to learn how to do it anyhow. But isn't it nice to know that you could?
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