A sharp (#) raises the pitch of a note by one semitone (one fret). A flat (b) lowers the pitch of a note by one semitone (one fret). So it is possible for a note on the fretboard to have two names, e.g., The note on the second fret of the 6th string can be called F sharp (F#) or G flat (Gb). Both names are correct but depending upon which key you are in only one name would be used. On each bar chord diagram the most used names of each note have been placed on the string containing the root note.
F# has the same position on the fretboard as Gb
G# has the same position on the fretboard as Ab
A# has the same position on the fretboard as Bb
C# has the same position on the fretboard as Db
D# has the same position on the fretboard as Eb
See Appendix Two for all the notes on the Guitar fretboard.