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A guitar fretboard notes diagram can get you started on understanding the neck of the guitar.
You can memorize the note names or you can explore the WWHWWWH pattern of a major scale which explains why this occurs not just on the guitar, but in music.
Above is a diagram of the fretboard. Notice that all the way to the left are the names of the open strings of the guitar.
When the guitar is sitting on your lap, the 6th string is the closest to you on the top but the lowest pitched note. The 1st string is the furthest from you on the bottom and the highest pitched string.
The circles indicate the notes in the key of C or all the white keys on a piano. This is the place to start in mastering the fretboard. The notes in between are the sharps and flats which occur in keys other than C.
To avoid confusion, you will notice that in between the circles where the sharp and flat notes exist, there are 2 names. Notes actually have two names. It depends on the key you are in. For example, F# is the same note as Gb. For an in depth understanding of enharmonic notes visit enharmonic and circle of fifths.
Another thing to notice is that on the 12th fret the guitar essentially starts over again. These notes are an octave up from the open strings. Play the open string, then push on this fret and play and you will hear the same note an octave apart.
This is also where you can hear the harmonic. If you just rest your finger gently on the 12th fret and play, you will hear the harmonic which is, again, an octave up.
Here is another free online guitar chord chart. In this diagram we can see the actual notes on the fretboard of the guitar. Frets 3, 5, 7, 9 and 12 are marked on the sides of guitars to help guitarists know where they are on the neck.
This visual gives us the guitar in context but is a little harder to see where those C major scale notes are. The first chart, I believe is better for that.
If you want to gain a complete understanding of the guitar visit my Learn to Play Guitar Lessons on this site...it's FREE!