Transposing instrument
A transposing instrument is a musical instrument for which music notation is not written at concert pitch (concert pitch is the pitch on a non-transposing instrument such as the piano). For example, playing a written middle C on a transposing instrument produces a pitch other than middle C; that sounding pitch identifies the interval of transposition when describing the instrument. Playing a written C on clarinet or soprano saxophone produces a concert B♭ (i.e. B♭ at concert pitch), so these are referred to as B♭ instruments. Providing transposed music for these instruments is a convention of musical notation. The instruments do not transpose the music; rather, their music is written at a transposed pitch. Where chords are indicated for improvisation they are also written in the appropriate transposed form.
For some instruments, a written C sounds as a C but is in a different octave; these instruments are said to transpose "at the octave". Pitches on the double bass sound an octave lower than written, while those on the piccolo and celesta sound an octave higher, and those on the glockenspiel sound two octaves higher.
Reasons for transposing[edit]
Ease of switching instruments[edit]
Some instruments are constructed in a variety of sizes, with the larger versions having a lower range than the smaller ones. Common examples are clarinets (the high E♭ clarinet, soprano instruments in C, B♭ and A, the alto in E♭, and the bass in B♭), flutes (the piccolo, transposing at the octave, the standard concert-pitch flute, and the alto flute in G), saxophones (in several octaves in B♭ and E♭), and trumpets (the common instrument in B♭, instruments in C, D and E♭, and the piccolo trumpet transposing at the octave). Music is often written in transposed form for these groups of instruments so that the fingerings correspond to the same written notes for any instrument in the family, even though the sounding pitches will differ. A musician who plays several instruments in a family can thus read music in the same way regardless of which particular instrument is being used.
Instruments that transpose this way are often said to be in a certain "key" (e.g., the "B♭ clarinet" or "clarinet in B♭"). This refers to the concert pitch that is heard when a written C is played on the instrument in question. Playing a written C produces a concert B♭ on a B♭ clarinet, a concert A on an A clarinet, and a concert C on a C clarinet (this last example is a non-transposing instrument).
Mechanical and physical considerations[edit]
Most woodwind instruments have one major scale whose execution involves lifting the fingers more or less sequentially from bottom to top. This scale is usually the one notated as a C scale (from C to C, with no sharps or flats) for that instrument. The note written as C sounds as the note of the instrument's transposition: on an E♭ alto saxophone, that note sounds as a concert E♭, while on an A clarinet, that note sounds as a concert A. The bassoon is an exception—it is not a transposing instrument despite its "home" scale being F.
Brass instruments, when played with no valves engaged (or, for trombones, with the slide all the way in), play a series of notes that form the overtone series based on some fundamental pitch, e.g., the B♭ trumpet, when played with no valves engaged, can play the overtones based on B♭. Usually, that pitch is the note that indicates the transposition of the instrument. Trombones are an exception: while tenor and bass trombones are pitched in B♭, and the alto trombone is in E♭, they read at concert pitch. This convention is not followed in British Brass Band music, where tenor trombone is treated as a transposing instrument in B♭. French horn is treated as a transposing instrument in F even though many horns have two (or even three) different sets of tubing in different keys (the common double horn has tubing in F and B♭).
In general, for these instruments there is some reason to consider a certain pitch the "home" note of an instrument, and that pitch is usually written as C for that instrument. The concert pitch of that note is what determines how we refer to the transposition of that instrument.