Tuning your clarinet

Things to make sure of first:
  • You need to use a good reed. A worn out or broken reed won't play in tune.
  • Your clarinet needs to be properly adjusted.
One tunes a clarinet by pulling out between the barrel and the left hand keyed joint. One can also pull in the middle, but this is kind of a last resort for instruments that are built out of tune. (However, see link below.)

If you pull out more than 1.5mm, use a barrel ring, or a spacing ring. A gap in the bore flattens the throat tones more than the rest of the instrument. You want to avoid a gap inside like this:
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Tune your clarinet paying particular attention to throat tone E and G. If these notes tend to be a hair flat when playing LOUD, it will help the rest of your clarinet to be better in tune. If your band tunes on concert B-flat, as is traditional (C on the B-flat clarinet), with a little practice you can hear how well you are in tune by playing the E and G against the band's B-flat. Remember, don't test the throat tones by playing softly, because they quickly go very sharp at soft levels.

See conflicting advice in this thread at woodwind.org:
Which joint to adjust when tuning
Generally, I don't recommend pulling out in the middle for instruments that have good intonation.
It is a must for Selmer Signets and other similar instruments. See my page on bore sizes.