If we look at a guitar fretboard we can see the notes are laid quite differently when compared to a piano:
The notes on a piano are presented to us in a more logic manner. Starting from the first note on the right side, the notes are in order and increase in pitch as we move from left to right.
This is only true to guitar if we were to play a single string and move along the fretboard in a linear fashion:
However due to the ergonomics of guitar and since we only have one hand to play notes with, a tuning method must be implemented in order for us to play both chords and scales with ease. The guitar is tuned from the low to high string - E A D G B E. The strings are tuned in fifths, meaning that if we have our first low E string and count up five semitones we will arrive at A:
If we count up five semitones from A we arrive at D:
From D to G is also five semitones:
But G to B string:
Wait...thats not a B?
If we were to follow the rule that each string is to be tuned a fifth above the previous the B string would actually be tuned to C and the final string would be F. The problem with this is that it would be difficult to play full chords that use all six strings, particularly useful if there is no other instrument within the band to play chords. So instead of five semitones from G our ancestors decided that it should be four. The last string however follows our original pattern and is five semitones away from B:
Cool. But why do I need to know that?
An understanding of how the guitar is tuned is very useful when learning scale patterns and shapes. If the strings were all tuned five semitones apart the scales would look symmetrical, but as we have this discrepancy between the G and B string, the shapes look asymmetrical. Once you understand this you can adjust the way you perceive scale and even chords shape and know that when you get to the B string you just move the pattern up or down a semitone.
I hope you have found this useful, let me know in the comments.
Taken from my book series 'Six String Enigma'
The AB Guide to Music Theory Vol 1
Chords And Scales For Guitarists
Ultimate Scale Book Pocket Guide Guitar Tab Book
Guitar Fretboard Fluency: The Creative Guide to Mastering The Guitar