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Understanding Walking Bass Lines
A major part of the Bass player's job is to (generally) "tell the truth". That is to say to "let the other musicians know what the chord is by playing the root on the 1 beat". (At least in a straight progression [eg. one that doesn't involve a downward line]).
The other part of the Bass player's job is to let the rest of the band know what the next chord is going to be.
A Bass player doesn't think: | 1 - 2 - 3 - 4 |.
A Bass player thinks: 2 - 3 - 4 - | 1 .
In other words beat 1 of bar 2 is the target note and the 2 - 3 - 4 before it are the notes leading the band into bar 2 beat 1.
To oversimplify the Bass player's job:
* Hit the root on the 1.
* Use a leading tone on the 4 (it can be either an upper leading tone or a lower leading tone. Example: you can approach a "target D" from either the C# [lower leading tone] or from an Eb [upper leading tone]).
* Beats 2 & 3 can pretty much be whatever you like, but most Bass players will prefer to use tones that are in the current chord.