In this quick lesson I share with you my approach to knowing which notes I am playing, regardless of a guitar's tuning.

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Memorizing each note on the fretboard is a tedious, almost painful task. So bad it seems to be, that most guitarists would gladly avoid it altogether if they could. The thing is, why would someone tell you to memorize every single note on the fretboard instead of memorizing the relations between all 12 notes, i.e. intervals?

Remember: all 12 notes of the Western temperament (C c# D d# E F G g# A a# B) stand in relation to each other by means of intervals. These intervals are static. A guitar's tuning is, however, prone to change in certain situations. What I am trying to say is that, while the tuning of the guitar might change, the interval between whatever note an open string represents and a given fretted note will always be the same. It is for this reason that I advice you to learn which intervals are found on which fret.

Let's start by identifying the most important intervals:
• The notes on the 12th and 24th frets will always be its octave.
• The notes on the 7th and 19th frets will always be the perfect fifth of the note to which the guitar string is tuned to.
• The notes on the 5th and 17th frets will always be its perfect fourth.
Now that we've identified these intervals we may continue with some more:
• The note on the 2nd and 14th fret will always be a major second.
• The note on the 3rd and fifteenth fret will always be the minor third.
• The note on the 4th and 1(six)th fret will always be the major third.
• The note on the 10th fret will always be the minor ninth.

Etc...

By approaching the fretboard in this almost mathematical fashion, you won't be confused by a guitar tuned to something other than standard tuning.

Perhaps this approach will prove helpful in your case. Even if not, you can still see how knowing a bit of music theory can prove useful at times.