Watch the Switch It Up online guitar lesson by David Santos from 50 Bass Grooves You MUST Know
This form is A B A B A.
The bass line is built from the G blues scale. It incorporates a minor third, Bb, in the main riff of Letter A even though the chord of the moment is a dominant 7th chord which contains a major third or a B natural.
Sometimes a blue note is a more appropriate choice, as opposed to the technically correct major note. The most important notes in a chord that are associated with it as being blue notes are typically the third and the seventh. A flatted third or flatted seventh played against a major chord are said to be blue notes. Sometimes the fifth can be flatted to give a blue sound to a line as well.
In this song the bass line in letter A contains a flatted 7th and a flatted 3rd. The groove is an 8th note pocket. Letter B goes to the four chord or a C7. The bass switches to a 16th note feel and utilizes a bass line that climbs from a low 3rd chromatically to the 5th of the C7 and then chromatically from the flat seventh to the root of the C7.