Watch the Roots On The Downbeats online guitar lesson by John Goldsby from Upright Bass Handbook
All great bass players share a common skill: they know how to outline harmony while keeping a groove. The early double bass players probably didn't know a lot about chords and scales, but they focused on nailing the roots of the chords and the harmonic movement, while at the same time making the band groove.
Today, bass players might know a lot about chords and scales, but our job hasn't changed much since the early days of jazz: We need to outline the root movement of a chord progression, and play in time with a strong groove. Theoretically speaking, all of the notes in any bassline can be analyzed as either being a (1) chord tone, (2) scale tone, or (3) chromatic passing tone or leading tone. That's the theoretical analysis, but in a musical analysis we should always trust out ears. If it sounds good, then it is good! Now let's look at how we can lay the groundwork for great basslines, and find our way around the lowest register of the bass on a 12-bar, Bb blues. Ready to swing? Let's play the blues!