Advanced · Chord Theory & the Number System

Understanding why chords work together lets you learn songs faster and play by ear.

▸ THE MAJOR SCALE
A key is built from its major scale (7 notes). In C: C D E F G A B.

▸ TRIADS
Stack every other note to build a 3-note chord (a triad). Built on each scale degree, you get the diatonic chords. In C major:
1 C 2 Dm 3 Em 4 F 5 G 6 Am 7 B°
Chords 1, 4, 5 are major; 2, 3, 6 are minor; 7 is diminished — this pattern is the SAME in every major key.

▸ THE NUMBER (NASHVILLE) SYSTEM
Name chords by their number, not their letter. A 1-5-6-4 progression is:
C–G–Am–F in C, or G–D–Em–C in G.
Once you hear progressions as numbers, you can transpose any song instantly and recognise patterns across thousands of songs.

▸ COMMON PROGRESSIONS
• 1-4-5 — folk, rock, classic
• 1-5-6-4 — pop "four chords"
• 6-4-1-5 — emotional ballads
• 2-5-1 — jazz cadence

▸ APPLY IT
Open a song on ReanChord, find the key, and label each chord with its number. Patterns will jump out.

Next: extended chords and voicings.