Instrument #9?

A clarinet (left) and Bb clarinet (right)

Today I picked up an A clarinet from a friend of mine from WCYO.  She has carpal tunnel and won't be able to play for a while, so I'll be taking over her part.  I'm the only clarinetist of the three of us who doesn't have both a Bb and an A clarinet, so I'm borrowing hers.

The only difference between a Bb clarinet and an A clarinet is the pitch (hence the names Bb and A.)  The fingerings and compass are exactly the same.  But because the A clarinet is longer, it sounds one half step lower.  Occasionally I have to fill in a part written for the A clarinet on my Bb clarinet.  In this case, I have to transpose down a half step.

So why was the A clarinet created if it's almost exactly the same as the Bb clarinet?  A clarinet is easiest to play in keys related to its own.  So, since "A" is a sharp key and "Bb" is a flat key, keys that are difficult for one type of clarinet are easy for the other.  Composers choose the type of clarinet based on the key they want their piece to be set in.  Here's a diagram of some key transpositions between the two clarinets:

Click to enlarge

Orchestral pieces that are written in sharp keys in concert pitch are best played on the A clarinet, and pieces in flat keys in concert pitch are best for the Bb clarinet.  I don't think I've ever seen concert band music that has more than 2 sharps for concert-pitch instruments.