Most regular chord progressions are made up of four bars. Like this:
|| G7. . . | C7. . . | G7. . . | D7. . . || x4
The 12-bar blues is different, instead of 4 bars per phrase. There's only 3, but since you play it 4 times. You still get a symmetrical sound.
12-bar blues (simple)
|| G7. . . | C7. . . | D7. . . || x4 (then do a different variation)
You can use any chords, but the most common are I, IV, V all as dominants or altered dominants.
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