There are seven letters (notes) in the musical alphabet; each fret position on the guitar has a letter name or an accidental attributed to it.
Starting on the open A string the letter names are A B C D E F G.
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Accidentals
Notes that don’t have letter names attributed to them are
known as accidentals. For example on
the A string the note between the first fret open A, and the second fret B is
referred to as A sharp B flat. The
symbol for a sharp is # and b
for flat
When moving from the open "A" to the first fret the note is referred to as A sharp. 
When moving to the first fret from the second fret "B" the note is referred to as B flat.
Below is a listing of all the notes on the A string including accidentals. The same pattern of notes repeats an octave higher starting at "A" on the twelfth fret. Note that there are no sharps or flats between B/C and E/F.
