This great question came from a subscriber who wanted to know how to go beyond very simple root notes when rehearsing and jamming with his garage band.
In this lesson, I'll teach you how to:
- Use your ear to figure out the key
- Map out a major scale on the fretboard
- Know how to play triads and arpeggios over each note of the scale
- Use the major pentatonic scale to create awesome fills and bass lines
What Key Am I In?
This is the first thing to work out because if you know the key then you know the scale and that's where all your bass line building blocks come from.
Check out these lessons:
Circle Of Fifths For Bass (How To Learn & USE!)
THE Most Important Skill For A Musician (& How To Learn It)
For this lesson, I used Bb Major as an example.

You can use this scale to link up notes from different chords. Speaking of which, the chords that come from Bb Major are as follows:
Bb Cm Dm Eb F Gm Adim
Musicians number them using roman numerals - upper case for major and lower case for minor:
I ii iii IV V vi vii
Bb Cm Dm Eb F Gm Adim
Now, if you don't know how to play those triads, watch this lesson:
Bass Guitar Music Theory: The 7 Arpeggios From C Major
It is absolutely central to you understanding how to do this! You learn ONE set of ideas that you can use in hundreds of songs.
Once you've got that down you can map out those arpeggios over this pattern.

Spend a fair bit of time trying to understand and memorise this. It's actually quite profound. In the above diagram and its underlying triads/arpeggios lies a whole world of bass lines ready for you to make up.
Incidentally, this makes recognising famous bass lines by ear much easier too.
What Notes Do I Play??
That all depends.
If you're playing rock, root notes work.
More melodic styles call for notes from the scale which sound tuneful when played together.
Roots, 5ths, and octaves, fill things out a bit.
Sevenths sound sophisticated.
Context is key and that comes from experience.
Listening helps so make sure to check out Easy Songs For Bass Players.
There is some theory involved in this. That's just the way it is. Spend some of your practice time memorising and getting your head around the homework and then some time bringing the ideas to musical life.
Do it until it's second nature and you'll soon see the magic.