I am definetely no expert but use scales- for example if a guitar is playing in the major scale just use the notes of the major scale. Obviously, you've heard of root note basslines, but to avoid them you can add a few other notes in (not in that scale) to add a bit of variation and difference between the bass and guitar.

Also, octaves are used a lot in basslines, e.g.


Triads can be used- i cannot fully explain a triad but maybe someone else will help.

Happy (bassline)writing
Just mess around mate - I only just got my bass but I've come up with some groovy-ass stuff.. I wouldn't have a clue about what scales are what on a bass, I just plod along with the rest of the guys. Sounds good though.

Doubt I've been of any help, but what can you do?
oh yeah, i dont know anthing about scales and octaves etc. even after all the threads and question ive made :P
^ Right now after all of the threads you've made, my only advice to you is get a teacher. People on the internet can't teach you as well as any teacher. Even for a couple of months just to get some scales and theory down.
Start by finding what key the music is in, then just improvise using the corresponding scale. So say if you are in the key of C major, play notes from the C major scale
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Also, octaves are used a lot in basslines, e.g.


Aren't those fifths? Octaves look like this:


and are very useful in writing basslines, especially slap bass. That's not to say don't use fifths. In fact, I use them more often than octaves, I find it gives more variety to the sound people actually hear, rather than the same note at different octaves.

But also, as many people have said, scales are your best friend in writing basslines. Some good starting ones are the Pentatonics and also the Blues scale (especially helpful if you're playing rock etc.).
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Man that's badass.

First off get a teacher or find a beginner workshop. You need to nail down scales, chords and chord progressions and basic rhythm patterns. There are some decent books and web sites out there, but really the best bet is a good teacher.
Think of bass as the glue between rythm and melody.

Start by figuring your rythm pattern, for this decide if your drummer has enough inspiration to lock on his kick, or if your drummer leaves a lot of space, with the tune in mind, *invent* your own kick line to surround your drummer's kick in a "call (you) - reply (drummer) - conclude (you)" pattern.

Once your happy with your groove, time to figure how to lay it down on the tune. Stick to the rythm chord progression only, eventually add a few fill-ins here and there, or come up with a second line to mix with the vocal line on some less busy parts.

Finally, without changing the groove, find alternatives to your main rythm line to play on second, third verses, and playing on the upper octaves for the last choruses as to build up.
As said, scales are the basis of music, so learn them. Once you learn some scales, even just major, minor and pentatonics you are laughing. Most common basslines are written in those scales. You just take those notes and jumble them up in a large bowl, crack in two whole grooves, add a pint of rhythm, a dash of flair and play over a hot metronome and you've got yourself a bassline. shamone.
Here are some ideas to help you learn how to write bass lines and play improvised ones as well.

1.Learn the melody of the song. This is very important. When improvising if you ever get lost then you'll have a backup plan at all times. Learning the vocal melody, guitar melody, sax melody, etc. also helps with creating bass lines. If you know the melody, you can incorporate it somewhat into your bass lines. Even just a 3 note line from the melody can make your bass line sound awesome.

2.LEARN YOUR SCALES AND TRIADS. Once you learn scales and triads you'll always have something you can play over a chord. Bass lines will suddenly become incredibly simple and easy to explain. You'll be able to take just a chord sheet and be able to make a bass line for it. Learning scales, triads, and keys are the main part to learning how to play bass lines, and once you do learn them, you'll be like "Ahhhh I get it now".

3.Listen to the drummer. Sometimes your bass lines will be lacking rhythm. It might be just quarter notes, eigth notes, etc. If this is the case, listening to the drummer's beats are always a good idea. Maybe he's playing a catchy rhythmic pattern. Then incorporate this into your line! The bass drum is a good place to start, then maybe go to the snare and the cymbals. If you get really good at this, then you and your drummer can actually play back ideas to each other and it really creates a cool effect in the music.

4.Order of note importance guideline. If you're just learning scales and you don't know which notes to hit, then follow this guideline on what notes you should play. Remember this is a guideline, and not a list of rules. So you don't have to follow this at all if you don't want to.

Order of Importance

Now looking at this you should see that roots are your most important notes, and that 13ths are your least important notes. The reason is you want your band or group to fill a large spectrum of sound. Basses should generally be playing in the lower part of the spectrum, drums should be in the middle, guitars should be somewhat middle-high in the spectrum and the keyboard/piano should be playing mainly highs and some middle spectrum stuff. The vocals should be able to be anywhere in the spectrum (except maybe the bassy/low part). If everyone played the root, it wouldn't sound so great. But if everyone plays different chord tones and they play in different ends of the spectrum then your music will sound great, diverse, and complex. Remember, this is a guideline. NOT A LIST OF RULES.

Hopefully you got something out of this post. If you want anymore help just ask
I think bass is the bridge from rythm to melody.

So with that said.

Learn triads, scales, and chords. Then you need to know about all that's going on with the drummer and the guitarist. Then you apply your knowledge of th triads, scales, and chords.
Bass is my life.
Another thing is that even if you're mainly just playing mostly root notes, you can "walk" down or up into the next root note using the notes of the scale the song is in.
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Would you do it?

If you could make everybody poor just so you could be rich,
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What would you do?
Aren't those fifths? Octaves look like this:

---5----7--- QUOTE]

Oh yeah, sorry i knew that octaves were along those lines i just forgot about the string spacing between them
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Start by finding what key the music is in, then just improvise using the corresponding scale. So say if you are in the key of C major, play notes from the C major scale

Exactly, this will help you make grooves that are a nice combonation of rhythm and melody. The more you noodle the more you'll get a feeling for what is best for what.