A triad is a three-note chord built by stacking two thirds on top of a root note. The four basic triad qualities are major, minor, augmented, and diminished. A major triad contains a root, a major third, and a perfect fifth, with four half steps between the root and third and three between the third and fifth. A minor triad uses a root, minor third, and perfect fifth, reversing the half-step distribution. An augmented triad consists of a root, major third, and augmented fifth and is built from two stacked major thirds, giving it a tense, unresolved quality. A diminished triad consists of a root, minor third, and diminished fifth and is built from two stacked minor thirds, sounding unstable and typically resolving to a more consonant chord.
Seventh chords extend triads by adding another third on top, producing four-note harmonies. The dominant seventh chord is a major triad with an added minor seventh, and contains a tritone between its third and ♭7th that drives resolution to the tonic. The major seventh chord is a major triad with an added major seventh, producing the smooth, lush sound characteristic of jazz, bossa nova, and R&B. The minor seventh chord is a minor triad with an added minor seventh and is the most common seventh chord built on the ii, iii, and vi degrees of a major key. The diminished seventh chord stacks a diminished seventh on a diminished triad and, because it consists entirely of stacked minor thirds, is perfectly symmetrical. The half-diminished seventh chord is a diminished triad with an added minor seventh, and frequently functions as the ii chord in minor keys.
Beyond seventh chords, extended chords add further chord tones above the octave. A ninth chord is a seventh chord with an added ninth, which is a second above the octave. Eleventh and thirteenth chords continue stacking thirds, adding the eleventh (a fourth above the octave) and the thirteenth (a sixth above the octave). In practice, musicians often omit certain tones, particularly the fifth and sometimes the eleventh, to avoid muddy voicings. Two other common chord alterations are the suspended chord, which replaces the third with the second (sus2) or fourth (sus4), creating an open sound that lacks major or minor quality, and the power chord, which contains only the root and perfect fifth, making it neither major nor minor and a staple of rock and metal music.
The order of notes in a chord can be rearranged through chord inversion, in which a note other than the root is placed in the bass. A triad in first inversion has the third in the bass, second inversion has the fifth in the bass, and so on. Inversions are often written using slash-chord notation, such as C/E for a C major chord with E in the bass. Inversions allow smoother voice leading and create different bass-line possibilities. The diatonic triads in any major key follow a predictable quality pattern: I (major), ii (minor), iii (minor), IV (major), V (major), vi (minor), and vii° (diminished), forming the foundation of Western harmonic function.