Transposition, and the music that changes pitch without changing

A singer asks for the song a tone lower. The saxophonist reads a part written in a different key from the piano's. A guitarist puts a capo on the second fret. All of them are doing the same thing: transposing, moving the whole piece by a fixed interval. The music does not change; only the pitch. If it sounds different, something was transposed wrong.

Every note, the same interval

To transpose is to add the same interval to every note. A melody in C major raised by a major second becomes the same melody in D major: every note went up a tone, without exception.

What is preserved is everything that matters: every interval between neighbouring notes, every chord, every tension and every resolution. The ear recognises the melody immediately, because the melody is the relationships, and the relationships did not change.

M2

The key signature goes with it

Transposing a piece is transposing its key, and the signature changes accordingly. C major raised a tone becomes D major, with two sharps; the melody's E becomes an F sharp, and the sharp is not a note-by-note decision: it is the new signature, applied to everything.

D major

The way to get it wrong is to transpose the notes and forget the accidentals. Raising the E to F, instead of F sharp, produces a minor second where there was a major one, and the melody comes out crooked. So you transpose by the key, not by the note: from the old signature to the new one, and the notes follow.

The spelling goes with it too

Real transposition preserves the letter, not only the sound. A melody with an F sharp, raised a semitone, has a G; raised a minor second, the F sharp becomes a G natural, because the minor second from F sharp is G. A diminished second, the interval that sounds like standing still, changes the letter without changing the sound: F sharp becomes G flat.

This looks like pedantry until the day a transposed part arrives full of double sharps because someone added semitones instead of intervals. Correct transposition adds named intervals, and the spelling comes out right by itself.

Transposing in your head: thinking in degrees

The musician who transposes on the spot does not calculate interval by interval. They think in degrees: the second degree is still the second degree in any key, the fourth-degree chord is still the fourth's. A chart read as "one, six, two, five" is played in any key with no transposing at all, because it was never read as notes.

CITonicAmviTonicDmiiSubdominantGVDominant

FITonicDmviTonicGmiiSubdominantCVDominant

The same sequence, C and F. Whoever read I–vi–ii–V played both without thinking; whoever read "C, A minor, D minor, G" had to calculate four times.

Transposing instruments

Some instruments read one note and sound another, for reasons of construction. The alto saxophone in E flat reads C and sounds E flat; the clarinet in B flat reads C and sounds B flat; the horn in F reads C and sounds F. Their parts are written already transposed, so the player always reads the same fingerings, and it is the arranger who transposes while writing.

M6

For the alto saxophone, the part is written a major sixth above the sounding pitch; for the clarinet and the tenor, a major second above; for the horn, a perfect fifth above. The rule is the usual one: a fixed, named interval, and the key signature that goes with it.

What to take from here

To transpose is to add the same interval to every note, and what is preserved is everything that makes the music itself: intervals, chords, tensions. The key signature changes with it, and so does the spelling, because an interval has a name and not only a size. Thinking in degrees is what makes transposition automatic, and transposing instruments are the case where the part arrives already transposed for whoever reads it.