transposition
The process of shifting all notes of a musical passage up or down by a constant interval
In Depth
Transposition preserves all the intervallic relationships within a piece while changing the actual pitches. It is used when a song is too high or low for a singer, when arranging for transposing instruments (B♭ clarinet, F horn), and as a compositional tool for developing musical material. Transposing instruments read in one key and sound in another — a B♭ trumpet reads C but sounds B♭. This historical convention persists because it allows players to use the same fingerings across different sizes of instrument.
Horn players must transpose constantly when reading older parts, which were written without a key signature — they need to calculate the interval for each different crook on the fly.