Function label

The same chord sequence can be described in two ways. With degree numbers you say “I–IV–V–I” and count which degree of the scale the chord is built on. With function labels you say “T–S–D–T” and describe what the chord does in the key. Functional theory assigns every chord to one of three main functions. The letters are the notation of German-speaking functional theory; English-language texts mostly use the degree numbers.

What the symbols mean

The tonic is the point of rest on which the music begins and ends. The dominant creates tension and resolves into the tonic. The subdominant lies on the other side of the tonic and usually leads on to the dominant. In the example above you hear the path from rest via the opening and the tension back to rest.

Capital and lower case

Capital letters stand for major chords, lower-case letters for minor chords: T is the major tonic, t the minor tonic. In minor one therefore usually writes t and s; if the dominant sounds there as a major chord with a leading tone, it gets a capital D. The basic principle stays the same in both modes: a symbol stands for a role, not for a chord name. Which chord takes over the role depends on the key.