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Octaves

Octaves are pretty useful when playing bass. If you didn't know, an octave is the same 
note, only higher, or lower.

Example.

G———5———      G——————————
D———————      D———2——————
A—3—————  or A——————————
E———————      E—0————————

The 3 and 5 are the same note; only the 5 (G) are higher. They are both the note 'C'
And the open E is the same as 2D. They are both E.

If you ever need to reach an octave, simply play the note 2 strings up and 2 frets up the 
neck.

(Note: 'Up' the strings, is actually down, if you don't understand, look at this)
             ^  G———————————————————————————————
             |  D———————————————————————————————
             |  A———————————————————1—2—3—4—5—6—
This is Up   |  E—1—2—3—4—5—6—7—8—9—————————————
                —— —— —— —— ——> And this is up.

The top string on bass (E) is referred to the bottom string. (You can remember this when 
thinking of tones, the open E string is a very low tone, so think of it as being on the bottom. 
And open G is a high tone, so think of it being on the top. Same with referring to fretted 
tones, the higher the fret you play, the higher the tone, hence — going up the neck)

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Octave Shapes

When playing with your fingers or a pick, you can easily reach an octave without hitting 
other notes. Use your index finger for the root, and your ring finger for hitting the 
ninth (octave). If thats too much of a stretch, use your pinky to reach the octave.

If slapping and playing the E string, you can still use the above info for octave shapes. 
But if playing the A string while slapping, its suggested that you mute the E string. 
To do this, put one of your fingers over the E string, but dont press it down                  
To complete the note. That way, when slapping you won't hit the open E. You can use 2 different 
fingers to mute it. You can use your index, or your middle. I use my index finger to mute. 
(Just so I don't accidentally press down on the A string) So I reach for my octaves using 
my middle finger and my pinky.

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Songs with octaves.

(Bracketed notes are octaves)

Aeroplane — RHCP:
G—————————————————(7)x—————————————
D—————(5)x—————————————————————————
A——————————————(5)—————4—3—————————
E——(3)—————5—6—————————————————————
    S  P P S S  S  P P S S
Still in Love Song — The Stills:
Chorus:
G—————————(8)—————(9)——————(10)————
D——————————————————————————————————
A—0000—(6)—————(7)—————(8)—————————
E——————————————————————————————————

Root — Deftones:
G———————(9)(9)————————77———————————
D—————————————99—1010————101099————
A—(7)(7)————————————————————————99—
E——————————————————————————————————

And thats my guide to octaves. =)