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Why Every Chord Is a Seventh
In an ordinary song in A major, you would expect a plain A major chord at home. The blues does not do that. It makes all three chords dominant sevenths: A7, D7 and E7.
A dominant 7th is a major chord with a flattened 7th added on top. On its own that added note is restless, unresolved, always leaning somewhere. The blues leans in every direction at once and never lands, and that tension is exactly the sound you are chasing.
This is also why you see track titles on this site written as "in A Major (A7)". The key is A, but the home chord is A7.
Learn these three shapes. The open voicings are enough to play a whole blues.
A7
A7 — the I chord, home base.
D7
D7 — the IV chord, arriving in bar 5.
E7
E7 — the V chord, the one that pulls you back to the top.
Play them in the order of the form from lesson one: four bars of A7, two of D7, two of A7, then E7, D7, A7, E7.
Don't worry about rhythm yet. One strum per bar is fine, and you can pause the track whenever a shape runs away from you. The goal is simply that your hand arrives on the right chord at the right time without you having to look.
Ray Charles Blues Guitar Backing Track Jam in A major
A Ray Charles-flavoured blues in A, built on exactly these three chords. One strum per bar until your hand lands on each shape without you looking.