If you play the 12 bar blues so that the tonic is just a plain triad until adding the minor 7th in bar 4 (which is actually how a lot of blues work) it is easier to see what happens.
With Roman numeral analysis under the chord letters:
A A A A7
I V7/IV
D D A A
IV I
G7 D A G7
V7 IV I V7
I left the seventh off of the IV
chord to keep the analysis simple.
In bar 4 you can see that adding the minor seventh changes the function of the chord and it becomes a secondary dominant to the IV
chord.
The reason this sounds so smooth to IV
is because it exploits one the most important voice leading concepts in western music: the scale degree FA
descends by half step to MI
.
In this case A7
to D
temporarily shifts the tonic to D
. The seventh of the A7
chord is the tone G
which is FA
in D
major, it descends to the F#
of the D
chord which is MI
in D
major.
It gets kind of wordy explaining exactly what happens and accounting for tonics, scale degrees, chord tones, etc. But that is the complete explanation.
- Secondary dominant
- Voice leading
FA
to MI