A diminished chord is formed by taking a major chord and lowering the third and fifth intervals by a half-step. Major chords consist of the root note and two major third intervals, while diminished chords pair the same root note with two minor thirds. The diminished seventh is a special variety of diminished chord that adds another minor third to the progression.
If the main melody of a piece of music is determined by individual notes, the backing harmony is determined by chords, which in turn are dependent on the key, or interval structure, of a piece. For example, a song in the key of C with no sharps or flats is based on the C major scale: C, D, E, F, G, A, B and the octave of C. This major scale provides the song with an upbeat-sounding key. Diminishing, or lowering, certain chords can change the tone of the song without having to rewrite the piece in a different key.
A major triad is formed by taking the root interval, the third interval and the fifth interval and playing them all simultaneously. The name of the chord is its root note; for example, the C chord always has C as its root note. The interval of a scale is the number of the note in relation to the root; in this example, E is the third interval of the C major scale. To form the diminished chord, two minor third intervals are used. A diminished C chord, written as Cdim in music notation, therefore contains the notes C, E flat, and G flat/F sharp.
Minor thirds always consist of three half-steps from the previous note, and are called minor thirds because of their role in forming minor scales, such as from C to E flat. In comparison, major thirds consist of four half-steps, such as from C to E. Minor chords have a more melancholy sound, but are still consonant. A diminished chord, on the other hand, sounds dissonant because of the relationship between the root and octave: two notes played a half-step apart will sound dissonant.
A special type of diminished chord is a diminished seventh, used to give a full, rich sound. It adds another minor third interval to the progression. In a C chord, a diminished seventh contains the notes C, E flat, F sharp and A. If another minor third were added, the next note would be the octave; the octave tends to override other tones in a chord.
Most often, a diminished triad's purpose is to provide a minor and dissonant sound, with the existence of two minor third intervals. A perfect fifth, such as C to G, has a balanced sound quality. Diminishing the note removes the balance of the sound as a whole.