This was of course impractical and also expensive. 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. 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). They can change their pitch all the time. For singers or violins this is no problem. Simplified speaking, an E in a C major scale would be a bit different from the E in A major.

The reason is that before 1700 the tuning of the notes was different in their scales from today. 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. 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.

They range from the small and high E-flat clarinet to the huge and low contrabass clarinet, and several other types in between. That instrument is by far the most commonly played clarinet, but there are many others in different sizes and forms. In the picture, it is the second from the left. When people talk about 'the' clarinet, they usually mean the Bb clarinet or the almost equally sized A clarinet. The Clarinet Family Different sizes from E flat to Contra Bass Clarinet

E flat major clarinet