Transposition

Transposing means moving a piece of music up or down by a fixed interval. Every note moves by the same amount, so the relationships between them survive and the tune is recognisably the same tune in a new key.

Why anyone does it

To fit a voice. A song sitting too high for a singer moves down a tone or three and becomes comfortable. This is the most common reason by far.

To fit an instrument. A piece written for one instrument may sit outside another's range, or fall into an awkward key for it.

To make it easier to play. Guitarists prefer some keys to others because of open strings; a piece in D♭ might be far easier in D.

Because the score demands it. Some instruments are written in a different key from the one that sounds — see below.

Two ways to do it

By interval. Decide the distance and move every note by it. C major up a major second is D major: C→D, E→F♯, G→A. You must transpose the interval, not just the letter, which is where accidentals come in.

By scale degree. More reliable in practice. Convert the music to degree numbers, then rebuild it in the target key. A progression that is I–V–vi–IV in C is C–G–Am–F; the same progression in G is G–D–Em–C. You never touch individual notes.

The degree method is faster, less error-prone, and has the useful side effect of teaching you the progression rather than the notes. See scale degrees.

The key signature moves too

Transposing changes the key signature, and the circle of fifths makes that lookup instant. Moving up a fifth is one step clockwise, so C major (no accidentals) becomes G major (one sharp). Moving up a tone is two steps clockwise: C becomes D, two sharps.

Transposing instruments

Some instruments are written in a key different from the one they sound. On a B♭ clarinet, a written C sounds as a B♭ — a tone lower than written.

The convention exists so a player can switch between instruments of the same family without relearning fingerings — the same written note gets the same fingering on every size of saxophone.

The consequence: a B♭ trumpet part is written a tone higher than concert pitch, so a band playing in concert B♭ reads a trumpet part in C.