Transposition · Music theory
Moving a melody to another key: by interval and by changing the clef.
What transposing is
To transpose is to write or play music at another pitch without changing anything else: melody, rhythm and mode stay the same, and every note moves by the same interval. It is done to fit a piece to a voice or an instrument.
The melody is instantly recognizable because its intervals do not change. What changes is the key, and with it the key signature.
Why we transpose
We transpose to fit the music to whoever performs it: a voice that cannot reach a note, an instrument with another range, a group with other possibilities. It is also an excellent exercise: it forces you to think in intervals and degrees rather than in keys on the piano.
A transposed melody keeps its identity because the ear recognizes distances, not absolute pitches: that is why anyone can sing a song without knowing its key. What changes is the color and the difficulty of playing it.