Lead-sheet symbols · Music theory
The letters and signs used to write chords in popular music and jazz, the suspended fourth, sixth chords and the bass after the slash.
Letters instead of degrees
A lead-sheet symbol writes each chord by its absolute name: a letter for the root, with its accidental, and some signs for the quality. It is the language of jazz and pop charts and of songs with chords, and it does not depend on the key: Dm7 is always the same chord, wherever it occurs.
Degree numbers, by contrast, say what role the chord has in the key. So the two complement each other: Dm7 is what the hand plays, and IIm7 is why. In this part of the site you will see them together, the chord above and its degree below.
One habit, two spellings
In countries that use solfège, both the letter and the note name are common: Dm7 and Rem7 are the same chord. Letters have taken over because they are shorter and travel well between countries.
In modern harmony, degrees are always written in capitals, and the sign gives the quality: IIm7, not ii7. It is the same convention this site uses throughout Theory.