Fingering a Fifth
You probably know it as a “1 by 2”: one string up, two frets over, like this (the numbers are fingering options for fifths):
According to my Wall Chart’s interval formula, this is a “1 by 2” – one string up, two frets over.
The Forgotten Fifth
But what about this one:
According to my Wall Chart’s interval formula, that is a “2 by -3”: 2 strings over, three frets back.
Across two strings you get your fifth covered between the pinky and the first finger. Totally one-finger-per-fret! And totally useful, especially for triads.
The two graphs above are taken from my book, Music Theory for the Bass Player, and the video that goes with this page, is this one:
Check this out:
When you play a 5th below the root you are actually playing the interval of a 4th.
That is because…
Fifths and fourths are inversions of each other.
I am sure you have done this: played a 1 – 5 groove with the 5th below the root. So you’d play C (let’s say that’s what the chord is) and the G (the 5th scale degree of C) below that C. Technically speaking the interval you just played is a 4th, however. Fourths and fifths are inversions of each other: in music 4 + 5 = 8 (inversions add up to the octave, which is 8)
Watch the CG ascending form a fifth, while the CG descending forms a fourth: 
Yes, you are playing the fifth scale degree if you go down to that G, but the interval you are playing is that of a fourth descending. If you were to play a fifth descending from the C, you’d end up at the F below.
It is correct to say you are “playing the fifth below” (meaning the G, which is the fifth scale degree of C major), but be aware that the interval you are playing is that of a fourth (so you are “playing a fourth down”. Sounds close when you say it in words, but to understand the difference is important.
Another way to think of it:
C is the root. G is the 5th. Where you play that G doesn’t matter. You can play it a 5th above or a 4th below that C. A bit of a different effect, but!
More on the fifth to come! Stay tuned!
