Transposing instrument
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A transposing instrument is an instrument which sounds at a different pitch (i.e. higher or lower) from the written music. For example: an instrument “in B flat†is an instrument in which a note is called (and written) B flat, but it sounds like a C. With an instrument “in A†an A will sound like a C (the name is always compared to a C).
Clarinets, saxophones and all brass instruments are transposing instruments.
Clarinets can come in C (a C is a C, so it does not transpose), but most clarinets are B flat clarinets. A clarinettist may be playing from music written in D major, but it will sound in C major. Music written in C major will sound in B flat major etc.
Professional clarinettists will need two clarinets: a B flat and an A clarinet. Some clarinet cases are made to hold both instruments. Some clarinettists also play the bass clarinet. This is also in B flat, but sounds an octave lower than the ordinary B flat clarinet. There is also an E flat clarinet which sounds a minor third higher than written.
Having transposing instruments makes it easier to play in lots of different keys. The fingering is always the same. A cor anglais is like an oboe but a fifth lower (a written C sounds like an F). Any oboist can play the cor anglais, reading the music and playing with normal oboe fingering. It will automatically sound a fifth lower (five notes of a scale). Compare this to recorders which are not transposing instruments. If you are used to a descant recorder you will know, for example, that the note played with 3 fingers of the left hand (1-2-3-0-0-0) is a G. If you want to play a G on a treble (alto) recorder you use fingering 1-2-3-1-2-3. This can be confusing until you are used to it.
Saxophones transpose into different keys according to their sizes. Brass instruments come in several different keys. It is always important to look at the music and to make sure you come to the rehearsal (and concert) with the right instrument. Some brass players are good at transposing i.e. if their music is written in the wrong key they can still play it in the right key. This is a useful skill.