Clarinet beginner here. Most Boehm clarinets have 17 keys, but you can get 18 keys for not much more money, and there are a few very expensive clarinets with 19 keys.
What do the extra keys do, and are they useful at a beginner to intermediate level?
Key 18 is apparently a left hand Eb lever, but which Eb? Do you get an extra semitone below the lowest E on a normal clarinet? Key 19 I have no idea except that it's an extra key for the right hand pinky.