A chord is three or more notes played together. The four basic triads (3-note chords) are built by stacking intervals of thirds.
Chords are built using tertian harmony — stacking notes in intervals of thirds (every other note in a scale). The type of third (major = 4 semitones, minor = 3 semitones) determines the chord quality.
Major Triad
Root + Major 3rd + Perfect 5th
Sounds bright and happy
Minor Triad
Root + Minor 3rd + Perfect 5th
Sounds dark and melancholy
Diminished Triad
Root + Minor 3rd + Diminished 5th
Sounds tense and unstable
Augmented Triad
Root + Major 3rd + Augmented 5th
Sounds mysterious and dreamy
C Major
Notes: C – E – G
Triad Formulas