Seventh chords and extensions (7, maj7, m7, 9)

The fourth tone: the seventh

A triad has three tones (1-3-5). If we add the seventh (the 7th degree), we get a seventh chord - a four-note chord with a richer, more colourful sound. Seventh chords are the foundation of jazz, soul and all more sophisticated harmony.

Three main types

• Dominant seventh (7) - major triad + minor seventh. Example: C7 = C-E-G-B♭. Tense, bluesy, calling for resolution.
• Major seventh (maj7) - major triad + major seventh. Example: Cmaj7 = C-E-G-B. Soft, dreamy, relaxed.
• Minor seventh (m7) - minor triad + minor seventh. Example: Cm7 = C-E♭-G-B♭. Warm, jazzy.

Why the dominant seventh sounds tense

In C7 the tones E and B♭ are a tritone apart - the most tense interval. That very tension gives the dominant seventh its strong urge to resolve toward a tonal centre (C7 → F).

Extensions: 9, 11, 13

If we keep stacking thirds above the seventh, we get extensions: the ninth (9), eleventh (11) and thirteenth (13). These are tones from the next octave that add colour to a chord without changing its basic function. Cmaj9 sounds even dreamier than Cmaj7.

Seventh chords and extensions are the ticket into advanced harmony. In the advanced tier we look at altered and extended chords that go a step further still.