Music Theory

Some Key and their Chords

   I   ii  iii   IV   V    iv  vii
  Maj min  min  Maj  Maj  min  dim
   C   d    e    F    G    a    b

   G   a    b

   D

   A

   E

shred music theory

(A correction to my table: the sixth chord should be written vi, the minor chord on the 6th degree, and in C it is A minor, not iv. The full pattern is I ii iii IV V vi vii°.)

The plain idea

Music theory is the grammar of music: names for the patterns that musicians already use, so that you can see why a song works and borrow its tricks. It doesn’t tell you what to write any more than grammar tells you what to say. It just tells you what you are doing.

Everything in Western harmony comes from three steps:

Twelve notes. The octave is split into 12 equal semitones: C, C♯, D, D♯, E, F, F♯, G, G♯, A, A♯, B, then C again. On a guitar, one fret is one semitone.
Seven of them make a scale. The major scale picks seven notes using the pattern whole, whole, half, whole, whole, whole, half (W W H W W W H). From C: C D E F G A B C. See Scales.
Stacking every other scale note makes chords. Take a scale note, skip one, take the next, skip one, take the next: C–E–G. That three-note chord is a triad. See Triads.

Why it matters

Knowing that most of the songs you love use the same handful of chords is liberating. Once you understand why the chords in a key belong together, you can learn songs faster, transpose them into a key that suits your voice, improvise over them, and write your own. Theory is a shortcut to hearing what’s going on.

The chords of a key

Build a triad on each of the seven notes of the major scale and you always get the same pattern of qualities:

Degree	I	ii	iii	IV	V	vi	vii°
Quality	major	minor	minor	major	major	minor	diminished
C major	C	Dm	Em	F	G	Am	B°
G major	G	Am	Bm	C	D	Em	F♯°
D major	D	Em	F♯m	G	A	Bm	C♯°
A major	A	Bm	C♯m	D	E	F♯m	G♯°
E major	E	F♯m	G♯m	A	B	C♯m	D♯°

Roman numerals let you talk about a song independent of key: capital for major, small for minor. “I–V–vi–IV” means C–G–Am–F in C, or G–D–Em–C in G.

Intervals: the distance between notes

An interval is the gap between two notes, counted in semitones. A handful are worth knowing by ear, because every melody and chord is made of them: the minor third (3 semitones) sounds sad or soft, the major third (4) bright, the perfect fourth (5) open and hymn-like, the perfect fifth (7) strong and stable (the “power chord” of rock), and the octave (12) the same note higher. A classic trick is to tie each to a song you already know: the opening of “Here Comes the Bride” is a fourth; the first two notes of the Star Wars theme a fifth; “Somewhere Over the Rainbow” leaps an octave. The Alphabet page shows the letter-name patterns for each interval.

Rhythm: the other half

Harmony gets the attention, but rhythm is half of music. Most popular songs are in 4/4: four beats to a bar, with weight on beats 1 and 3, while the backbeat, the snare on 2 and 4, is what makes rock, pop and soul feel the way they do. Waltzes and many folk songs are in 3/4. Learning to count, clap and tap along before you play is the quickest way to sound musical rather than merely correct.

An everyday analogy: home, away, and the trip back

Think of the I chord as home. The IV chord is a walk to the neighbour’s: pleasant, a little away. The V chord is standing at your own front door with the keys in your hand: it wants to resolve to I. Almost every song is a journey that leaves home and comes back, and the V→I move is the “coming home” feeling. That pull is called a cadence.

A worked example: why the V chord pulls home

In C major, G7 is G–B–D–F. Two of its notes are only a semitone away from notes of C major: B wants to rise to C, and F wants to fall to E. Those two little semitone pulls, the leading tone and the seventh, are why G7→C sounds so final. Play G7 and stop; you’ll hear the tension. Then play C. That’s resolution, and it’s the engine of Western harmony.

Common misconceptions

“Theory is rules you must obey.” It describes what tends to work. Great music breaks “rules” constantly, usually on purpose.
“You need to read sheet music to learn theory.” Chord symbols, Roman numerals and your ears go a very long way.
“Theory kills creativity.” It gives you more options, faster. Many self-taught greats knew far more theory than they claimed, they just called it by other names.

How it connects

Scales, Triads, Chord Progressions
Musical Keys, D Major, The Alphabet, Solfege
Learning the Fretboard and Songs to Learn
Groups: the twelve notes form the 12-hour clock group $\mathbb{Z}_{12}$ ; transposing is adding.

The truth revealed: twelve notes, one scale pattern and one stacking rule build almost every song on the radio.