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
D D A A
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
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
In this case
D temporarily shifts the tonic to
D. The seventh of the
A7 chord is the tone
G which is
D major, it descends to the
F# of the
D chord which is
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