What they are
The thirds and sevenths of chords are the notes that define their quality (major, minor, dominant). Connecting them from one chord to the next with minimal motion produces smooth lines, by step or by common tone: guide tone lines, one of the pillars of Berklee harmony courses.
Example in C: Em7 – A7 – Dm7 – G7 – Cmaj7
- Em7: G (3), D (7)
- A7: C# (3), G (7)
- Dm7: F (3), C (7)
- G7: B (3), F (7)
- Cmaj7: E (3), B (7)
Line 1: D → C# → C → B → B (chromatic descent).
Line 2: G → G → F → F → E.
Over a cycle of fifths, thirds and sevenths always swap roles: the seventh falls by a half step and becomes the third of the next chord.
Uses
- Arranging: strings, horns or choirs playing only the guide tones over a bass: complete harmony with two voices.
- Piano: three-note voicings (root + 3 + 7, or 7 + 3) for comping.
- Melody and improvisation: aiming for thirds and sevenths on chord changes makes the melody sit “inside” the harmony.
- Background vocals: two voices singing “ooh” on the guide tones under the lead.
Real-world example
“Autumn Leaves” is the classic exercise for guide tones: over its ii–V chains the thirds and sevenths form two almost chromatic descending lines, which many arrangers give to horns or strings.