The Clarinet Family
Different sizes from E flat to Contra Bass Clarinet
When people talk about "the" clarinet, they usually mean the Bb clarinet or the almost equally sized A clarinet. In the picture, it is the second from the left. That instrument is by far the most commonly played clarinet, but there are many others in different sizes and forms. They range from the small and high E-flat clarinet to the huge and low contrabass clarinet, and several other types in between.
Some, like the B-flat and A clarinet, can be found in almost every orchestra, and they appear in most scores after 1770, later also E-flat and bass clarinets, but others like the basset horn are only found in a handful of compositions.
However back in 1700 it common for all instruments with fixed tuning that different sizes and types of tuning were normal, it was required because of the old scales. The reason is that before 1700 the tuning of the notes was different in their scales from today. Simplified speaking, an E in a C major scale would be a bit different from the E in A major. For singers or violins this is no problem. They can change their pitch all the time. Wood, however, must have fixed tunings. So they could only play music in one key or in keys closely "related" to it (for Bb instruments, that would be F major and E flat major).
So all woodwinds, including clarinets, had to be made in many different keys: There were A- B flat- C- D- and E-flat clarinets and so on. This was of course impractical and also expensive. With Johann Sebastian Bach the tuning changed a little bit: He tuned the piano and later all other instruments in a balanced way so the E in C major and the E in A major would be the same. There is a tiny deviation from "perfect" harmony, but the practical benefit was so immense, that today all western instruments are tuned like that.
In result also a much smaller number of instrument types was needed, no more C and D clarinets... The smaller number of instrument types have become established in orchestras among professionals and even more so among amateurs. Composers after 1700 practically only write for these types. Only rarely will one experience another instrument in concerts today. Many older works are rearranged for these types because objectively, for the listener, there is hardly any difference to be heard between G-sharp major and G-major in the modern, tempered tuning, except that the musicians can play much more relaxed and therefore they can focus on producing a better harmony. The remaining set of clarinets now is the modern clarinet family that you see in the picture. All clarinets except the counter base are quite common in orchestras today.
You will find a detailed description of each type on its own page:
The clarinet is a transposing instrument. In the beginning there were clarinets for nearly all scales, but today only B flat, A, and E flat remain (yes, I know there are G / C instruments). If a composer asks for a C clarinet you will have to transpose the notes for your B flat or better for your A clarinet, if you have got one.
Rarely used sizes
The instrument types above are the common ones for the classical symphonic orchestra and harmony orchestras (including marching bands and big bands) of western culture. In other cultures things look different: The traditional Turkish orchestra - which by the way plays music that is harmonically and rhythmically much more complex than what we see in the European style orchesters - employs different clarinets, for example the clarinet in G.
One reader - Thomas Aigner - writes: "... next to that there are high-G and high A flat clarinets. They are used in Viennese folk music (called "Schrammel-Musik"). In contrast to the E flat clarinet the high-G has a very fine sound. You can have this built by Schwenk und Seggelke, were you find a description of the instrument."
Can you replace different types of similar size with each other?
This is a common question: Isn't it possible, to play - instead of an E flat clarinet - a B flat clarinet, just an octave higher than it's notes are written? Then you could save the money for the E flat clarinet, and it would be enough to have, say, a B flat and a bass clarinet (B flat, too) for all existing literature. You do play the same notes then, don't you?
The answer is NO - this won't work well, at least not under all conditions. In a tutti part, when everybody plays fff, and just the complete harmony is important, it may be difficult for the audience to notice that you used a different instrument type. When you have a more prominent part though, the difference becomes more obvious: The clarinet's sound differs strongly in three registers of the instrument. Notes that an E flat clarinet plays in the sonore lower register with strong low overtones would be played in the rather neutral part of the upper register of an A or B flat clarinet. The clarinet is not a cheap synthesizer where every note sounds the same, just at a different frequency. If the composer wanted this effect, and the audience expects it, you better come close to what is expected. In case of emergency you may play a part with another clarinet of similar size (replace A with B flat or bassett horn with alto clarinet, but then you will have a transposition problem with lots of sharps or flats. So if you have to replace one clarinet with another, keep in mind that it will sound differently. Of course it is still better to play the E flat clarinet's part with a B flat clarinet than with a flute.