intervals, with each such third being either major (M3) or minor (m3). More triads: diminished and augmented As discussed above, major and minor triads are constructed by stacking thirds: The major triad concatenates (M3,m3), supplementing M3 with a perfect-fifth (P5) interval, and the minor triad concatenates (m3, M3), supplementing m3 with a P5 interval. Similar tertian harmonization yields the remaining two triads: the diminished triad concatenates (m3,m3), supplementing m3 with a diminished-fifth interval, and the augmented triad concatenates (M3,M3), supplementing M3 with an augmented-fifth interval. More sevenths: major, minor, and (half-)diminished Stacking thirds also constructs the most used seventh-chords. The most important seventh-chords concatenate a major triad with a third interval, supplementing it with a seventh interval: The (dominant) major-minor seventh concatenates a major triad with another minor third, supplementing it with a minor-seventh interval. The major seventh concatenates a major triad with a major third, supplementing it with a major-seventh interval. The minor seventh concatenates a minor triad with a minor third, supplementing it with a minor-seventh interval. The half-diminished seventh concatenates a diminished triad with a major third, supplementing it with a diminished-seventh interval. The (fully) diminished seventh concatenates a diminished triad with a minor third, supplementing it with a diminished-seventh interval. Four of these five seventh-chords—all but the diminished seventh—are constructed via the tertian harmonization of a major scale. As already stated, The major-minor seventh has the dominant V7 function. The major seventh plays the tonic (I7) and subdominant (IV7) roles; The minor seventh plays the ii7, iii7, and vi7 roles. The half-diminished seventh plays the vii7 role. While absent from the tertian harmonization of the major scale, The diminished seventh plays the vii7 role in the tertian harmonization of the harmonic minor scale. Besides these five types there are many more seventh-chords, which are less used in the tonal harmony of the common-practice period. When playing seventh chords, guitarists often play only subset of notes from the chord. The fifth is often omitted. When a guitar is accompanied by a bass, the guitarist may omit the bass note from a chord. As discussed earlier, the third of a triad is doubled to emphasize its major or minor quality; similarly, the third of a seventh is doubled to emphasize its major or minor quality. The most frequent seventh is the dominant seventh; the minor, half-diminished, and major sevenths are also popular. Chord progression: circle of fifths The