Okay, so, um... how does one actually begin to write their own bass lines?
Sorry for the stupid question; Thanks in advance.
Someone should really sticky a bassline help thread. I'll start off on the basics and you can add.

First you should start off by deciding what type of music you want to play. This will help you chose your scales and modes. I'll divide the 7 modes into genres which are played (usually) through these modes.

A mode is a series of intervals which define a certain pitch. For instance an interval is just measurement of a musical distance. So dealing with modes it would just be half step or whole step. A half step is the distance from any key to the key immediately above or below it. For example going from F to F# should be a half step. A whole step is going from one key to another key and having another key between them. For example going from F to G.

Ionian Mode (W-W-H-W-W-W-H) Pop, Modern rock, etc.
Dorian Mode (W-H-W-W-W-H-W) Country, folk music, goth,
Phrygian Mode (H-W-W-W-H-W-W) Good for solos, angry music,
Lydian Mode (W-W-W-H-W-W-H) Jazz; gives sense of happiness to listeners
Mixolydian Mode (W-W-H-W-W-H-W) Blues; epic
Aeolian Mode (W-H-W-W-H-W-W) Metal; gives strong sense of sadness in the song.
Locrian Mode (H-W-W-H-W-W-W) Metallica really popularized this mode. Before the mode was just thought to be theoretical.

Then choose your root note and scale ; if you want metal just go with the trusty Pentatonic Blues Scale and Aeolian Mode. Be creative add pull off, slides, or whatever. But this is just the tip of the iceberg, so hope this helps a little.
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wow you need to get into more music theory. There is far more to scales than just the box.

I'll do my best though.

Before you learn anything, you need to learn intervals. In order to learn intervals, you really need to learn the major scale.

The major scale goes Root, Major 2nd, Major 3rd, Perfect 4th, Perfect 5th, Major 6th, Major 7th, Octave.
In the key of G it will look like this:

R M2 M3 P4 P5 M6 M7 Octave

The names of all intervals can really be determined by the function in the major scale. That statement may be really over your head, but let's do it like this.
The fourth and fifth, without all that theory nonsense, are the 2 coolest intervals to play with. Why? Well scientifically because of how their soundwaves bond, they match up at very even points (root goes by twice, intervals goes by 3 times; root goes by three times, interval goes by twice). Non-scientifically, they just do, alright? Because of these bonds where they match up so evenly, they are called "perfects".
The fourth, the fifth, and hte octave are all called "perfects" because they all share this.

Everything else (in the major scale)'s interval title will be called major. Why? Because everything else has some purpose tonally. That statement may have been over your head. So I'll put it this way: the 4ths, 5ths, and octaves are there to make it cool. The other intervals are there to give it a feel. If I play a bassline just root, fifth... it could be sad, it could be happy. No one knows damn it! If I play a bassline thats root, major third, perfect fifth, it's probably at a happier moment in the song.

So. What I'm saying is, if it's not a perfect, it is there for... emotion I guess. The better way to say it is how I originally did: everything else has a purpose tonally. Because most other music theory is based on the major scale, these intervals in the major scale are all called majors.

To get the names of other intervals, use this formula. (If you didn't know, a half step is just one note/1 fret/smallest distance in sheet music.)

Use this formula

Diminished <- Minor <- Major -> Augmented
Diminished <- Perfect -> Augmented

<- means move it a halfstep lower. -> means move it a halfstep higher.

So, C major scale is C D E F G A B C.
The major 7th is B. Because its the 7th note in the major scale.
The minor 7th can then be figured out by lowering the major 7th a halfstep.
The major 7th is B. B down a halfstep is Bb. Bb is the minor 7th of C.

You can do this for any interval as long as you know the theory to get the interval correctly.

I must also state, this is why A# and Bb are different notes. Yes, they sound exactly the same. But an easy way to spot a musical dumbass is by letting him say something like A# in a situation where it asks for Bb.
A# is an augmented 6th. Because A is a major 6th, and A# is A raised once. That is why A# is an augmented 6th. It is not a minor 7th.

Hope that didn't confuse you.

Anyways, to get to the real point now that the basics are out of the way:
A guitar (or really most instruments that don't serve the purpose as the bass) usually plays chords. Chords are 3 or more notes played at the same time.

There are four kinds of triads (triads = 3 basic notes that most chords are formed off of).
We'll start with the two you've heard about.
The major triad (Root, Major 3rd, Perfect Fifth), and the minor triad (Root, minor 3rd, Perfect 5th.)
A major chord is just a major triad.
A minor chord is just a minor triad.
The other two triads are diminished (Root, Minor 3rd, Diminished 5th), and augmented (root, Major third, augmented 5th). These aren't very common because they don't... they're harder to use because they don't have that perfect 5th in it that makes it easy to listen to.


The major scale always goes Root Major 2nd Major 3rd Perfect 4th Perfect 5th Major 6th Major 7th Octave.
This means we can figure out the chords in a key just by adding the proper notes.


We obviously know the C major triad is the C triad in the major scale, but what about D?


D F A. F is a minor third interval for D, and A is a perfect fifth interval to D. Thus, D forms a Dm triad in C major.

Using this, and roman numerals, I can quickly explain all the chords in a key. The numeral indicates which note in the scale it is, if its capitalized its major, if its lowercase its minor. If its lowercase and says diminished, its diminished. lol.

I ii iii IV V vi viidiminished

and there you have chords within in a key.

This is why "discovering the key to a song", although seeminglessly useless unless you're writing sheet music to the first timer, is very hand when writing a song. Don't want to play the root?
Stay within in the key and dance around the root, or the other parts of its chord.

There's billions of other pieces of music theory I could explain to you, but I thought this was some of the most basic, & helpful. And I was in a good enough mood to explain it so I did.

If you have any questions ask. Odds are that went way over your head.

PS. the simple way to understaning modes.
The 7 modes everyone refers to as "the modes" (even though there are far more modes because there are scales other than the major) are the modes of the major scale.
They are simply a given major scale, with a different root.

Huh? how do I mean?

I mean C major is C D E F G A B C. So the mode of it in D would be D E F G A B C D.
Past that, its a matter of learning the names of all the modes, and then learning which one does what.

I: Ionian
ii: Dorian
iii: Phrygian
IV: Lydian
V: Mixolydian
vi: Aeolian
viidiminished: Locrian.

I hope I didn't go over everyone's head.
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When i think of writing bass lines, i take the metallica approach and jam some random riff, and base the whole song around it ^_^. Or, i just bang heads with a fellow guitarist for ideas and we both make a song.
Originally Posted by *****bass
you know he died right???


^i usually just riff along also. Use some modes for when i'm creating fills for inbetween songs. And Dmajor scale. Don't know why but i like that scale.
^ the essence of music mate !

Scales are there to tell you what notes you can play, dont get to caught up in the repetition of them - your basslines can be become very scalic. You need to "feel" your bassline as much as think about it.