One, Four and Five

Of the seven chords in a key, three do most of the work: I, IV and V.

They matter because of where their notes sit. I is home — stable, finished, the chord a song ends on. V is the least stable, and it contains the scale's seventh note, which sits only a half step below the tonic and leans on it hard. That lean is why V wants to resolve to I so badly. IV is in between: it moves away from home without the tension of V.

Put a flattened seventh on top of the V chord and the pull gets stronger still. That is what a dominant seventh is, and it is why V7 to I is the most common ending in music.

In G major, I is G, IV is C and V is D. Three chords, and an enormous amount of blues, country, rock and pop is nothing else.

G

G — the I chord. Home.

C

C — the IV chord. Away from home, but calm.

D7

D7 — the V chord with a seventh on top. Listen to how badly it wants to become G.

This is a straight twelve-bar blues in G, and the twelve-bar blues is the most famous I IV V form there is. Its skeleton: four bars of I, two of IV, two of I, then V, IV and back to I.

Play along using only those three chords, and count the bars as they go. You will hear the form turn over every twelve bars whether or not you are counting correctly, which makes this an unusually forgiving place to practise counting.

Pay particular attention to the moment the D7 arrives near the end. That is the tension the whole form has been building, and hearing it land on the G is the sound of theory doing something audible.

A straight twelve-bar blues in G. I, IV and V, and nothing else.