‘Sister Sadie’ is sort of the reverse of ‘Moanin’ (Link) – a “response and call” if you will. The song makes good use of a pentatonic scale.

One of the key features of this 5 note scale is its avoidance of tension.

The Major Pentatonic avoids both the 4th and 7th degrees. The so called “Soul Pentatonic” also avoids the 2nd, while adding the “bluesy” b3. ‘Sister Sadie’ is a classic example of this soul pentatonic, so much so that some people call it the “Sister Sadie Pentatonic”.

Pentatonic scales predate the Western major scales by many hundreds of years and are used in virtually all cultures of the world.