Triads

A triad is a chord of exactly three notes: a root, a third above it, and a fifth above it. In degree numbers, a triad is always 1 3 5: the root is 1, then we skip a note to reach 3, and skip another to reach 5.

The pattern is "play one, skip one". Starting on C in the C major scale: play C, skip D, play E, skip F, play G. Play those three notes one after another and you hear an arpeggio; play them together and you hear a chord.

The starting note can be any degree of the scale, not only 1. Start the same skip-one pattern on D and you get D F A; start on G and you get G B D. Same shape, different notes, and, as we will see, sometimes a different mood.

A major triad is degrees 1 3 5 of the major scale built on its root. It is the bright, open, settled sound most people picture when they hear the word "chord".

Name the two intervals and you have named the chord: the 3 is a major third above the root and the 5 is a perfect fifth above it. The major third is what makes the chord sound major. Notice that D major needs F#: a major third above D is F#, while the plain F is only a minor third, one step too dark.

A minor triad is a major triad with the third lowered: 1 b3 5. The major third becomes a minor third. One note moves, and the whole mood turns darker and more inward.

The b3 is a minor third above the root, one semitone smaller than the major third. The 5 stays a perfect fifth. So the only difference between major and minor is the middle note, and that is why we say the third decides the mood: a major third sounds major, a minor third sounds minor.

A diminished triad is a minor triad with the fifth lowered as well: 1 b3 b5. Both upper notes are pulled in toward the root, and the result sounds tense and unstable: a chord that wants to move somewhere.

The b5 is a diminished fifth above the root, one semitone smaller than the perfect fifth. The chord is two minor thirds stacked (B to D, then D to F), which is why nothing in it feels like solid ground. In a major key this triad appears on degree 7, and it resolves most naturally up to the tonic chord.

An augmented triad goes the other way: keep the major third and raise the fifth, 1 3 #5. The perfect fifth becomes an augmented fifth. It sounds bright but suspended: floating, never quite landing.

The #5 is an augmented fifth above the root, one semitone larger than the perfect fifth. The chord is two major thirds stacked (C to E, then E to G#). Stack one more major third on top of G# and you land back on C, so the notes are spaced perfectly evenly around the octave: C E G#, E G# C and G# C E contain exactly the same notes, and the ear cannot tell which one is the root. That symmetry is where the floating quality comes from.

There are exactly four triad qualities, and they differ only in how the 3 and the 5 are adjusted from the major-scale positions. Put them on one root and the family is easy to see.

A useful way to hear them: major and minor share a perfect fifth, so they both sound stable, and the third just chooses bright or dark. Diminished and augmented have an altered fifth, so they both sound unstable: squeezed in one case, stretched in the other.

Now the key idea of the chapter: start on each note of a major scale, stack the skip-one pattern 1 3 5 using only notes of that scale, and you get the seven chords that belong to that key. These are the diatonic triads.

No new notes appear: every chord is made only from C D E F G A B. That is what makes these seven chords sound like they belong together. Play them in any order and you never leave the key of C major.

Stacking 1 3 5 on every degree does not give seven major chords. The qualities come out in a fixed pattern, major, minor, minor, major, major, minor, diminished, and that pattern is identical in every major key.

Why does it fall this way? The major scale has two places where neighbouring notes are only a semitone apart: E to F and B to C. A third that contains one of those small gaps is a minor third; a third that avoids both is a major third. Chords on 1, 4 and 5 avoid them and come out major; chords on 2, 3 and 6 catch one and come out minor. The chord on 7 is the only one whose fifth (B to F) contains both gaps, so it shrinks from a perfect fifth to a diminished fifth.

Three of the seven diatonic triads do most of the work in a key: I, IV and V, the primary chords. They are the three major triads, and between them they contain every note of the scale.

These functions come straight from the degree names you learned in the scales chapter: the chord built on the tonic is the tonic chord, on the subdominant the subdominant chord, on the dominant the dominant chord. The V chord is the strongest pull because it contains the leading tone (B in C major), which wants to rise to the tonic.

Musicians write triads in two shorthands. Chord symbols name the actual chord (Em); roman numerals name its position in the key (iii). You will see both constantly, so let's fix them now.

Roman numerals follow the same logic with case instead of letters: upper case for major (I IV V), lower case for minor (ii iii vi), a ° for diminished (vii°) and a + for augmented. The advantage of numerals is that they are relative: I IV V means the same thing in every key.

Put the two together and you can read a chord chart. Am-F-C-G in the key of C major is vi-IV-I-V: four diatonic triads, no notes outside the scale.

Because the pattern I ii iii IV V vi vii° is fixed, you can build the chords of any major key by stacking 1 3 5 on its scale. Let's do the two keys next to C: G major (one sharp, F#) and F major (one flat, Bb).

Watch where the accidental lands. In G major the F# appears in three chords, iii, V and vii°, and the Em on vi is the same chord as iii in C major. In F major the Bb appears in ii, IV and vii°. The chord C E G is I in C, IV in G and V in F: same notes, three different jobs.

Everything we do with harmony from here on starts with what you now know: a triad is 1 3 5, it comes in four qualities, and a major key gives you seven of them in the fixed pattern I ii iii IV V vi vii°.

In the next chapter we put the seven diatonic triads in motion and learn why some orders of chords feel like a journey home while others feel like a question. Later chapters will turn triads upside down (inversions) and add a fourth note (seventh chords). Notice that none of those ideas replaces the triad. They all grow out of 1 3 5.