The 12-bar blues form
The twelve-bar structure you solo over, the I, IV and V, and where the changes actually land.
Twelve bars, three chords, repeating. The form is fixed enough that a room full of strangers can play it together without discussing anything, which is exactly why it is worth knowing cold. The grid above takes the common quick change: bar 1 is the I, bar 2 goes straight to the IV, and bars 3 and 4 come back to the I. Bars 5 and 6 are the IV, bars 7 and 8 the I. Bar 9 is the V, bar 10 the IV, bar 11 returns to the I, and bar 12 is the V that sends the whole thing round again.
The bars that matter most for a soloist are the ones where the chord changes. Everything can sound fine over the I and then fall apart at the big arrival of the IV in bar 5, because that is the moment your note choice is exposed. Practising the change itself — the last beat of one chord and the first of the next — is worth more than running scales through the whole form.
The last two bars are the turnaround: the I falling back to the V, which winds the form round to bar one again. It is what stops twelve bars from merely stopping, and the reason a blues can go round twenty times and still sound like it is heading somewhere.
How to count it
Count it in four-bar sentences rather than in twelve bars. Bars 1 to 4 state something. Bars 5 to 8 say it again over the IV. Bars 9 to 12 are the climb and the way home. Sung blues works exactly this way — a line, the same line repeated, and a third line that answers both — and once you hear the form as three sentences instead of twelve bars you stop losing your place in it.
The quick change in bar 2 is the one thing to listen for at the top. Some bands play it and some stay on the I for four bars, and if you assume the wrong one your bar 2 will disagree with everybody else's. It is audible on the first chorus, so listen before you commit.
What to play over it
One scale gets you through the whole thing: A minor pentatonic covers all twelve bars in A, which is why it is where everybody starts and why so many players never leave.
Two devices take you past that. The first is target tones: each chord has a major 3rd that is not in the pentatonic, so aim at the C♯ while the A7 is under you, the F♯ while the D7 is, the G♯ while the E7 is. The second is switching scale with the chord, playing A Mixolydian, then D Mixolydian, then E Mixolydian. They are one idea at two levels of ambition — a single note following the harmony, or the whole set of notes following it.
The form transposes without changing shape. In G the three chords are G7, C7 and D7; in E they are E7, A7 and B7. The bar numbers never move, which is why a blues is the one form you can walk into cold.
The common mistake
Soloing without counting. It is entirely possible to play well for eight bars and have no idea you are in bar 9, and the tell is a phrase that runs straight through a change as though nothing happened underneath it. The fix is not to count harder; it is to play the chords. Comp the form yourself for a few days, and bar 5 stops being a number you are tracking and becomes something you can feel coming. A player who has played the rhythm part knows where they are in the form without doing any arithmetic at all.