The Circle of 5ths dictates how many sharps (#) or flats (b) is contained in a given key. For most people learning popular and folk guitar styles you probably don’t need this information anytime soon. It is more likely to be relevant if you’re reading sheet music (not tab), and into classical music or jazz.
Here is a quick reference for navigating the Circle of 5ths (for sharp/# keys), or the Circle of 4ths (for flat/b keys) on the fretboard:
For the Circle of 5ths, start on the C note on the 3rd fret of the A string as shown in the first illustration:
1) C
2) G – #
3) D – ##
4) A – ###
5) E – ####
6) B – ### ##
7) F# – ### ###
To find flat keys we use the Circle of 4ths, starting on the C on the 8th fret of the low E string as shown in the second illustration:
1) C
2) F – b
3) Bb – bb
4) Eb – bbb
5) Ab – bbbb
6) Db – bbb bb
7) Gb – bbb bbb
Here is the mirror image for anyone playing a left-handed guitar:
How do we know what the new sharp or flat is?
For sharp keys, the new sharp is the note one fret down from the new key tonic.
For flat keys, the new flat is the next note in the cycle of 4ths you are currently navigating.
I can explain this more thoroughly if needed. Just bookmarking the concept here for now.