Because each fret on the fingerboard marks the distance of a semitone, wherever a string is stopped at any fret, the next fret stopped either above or below is a semitone, as it is from an open string to the first fret. Therefore due to the nature of the guitar’s tuning, by stopping the sixth string at the fifth fret, we obtain the A which is a unison of the open fifth string A. Similarly, if we stop the sixth string at the sixth fret, we obtain the A# (Bb) which is a unison of the fifth string, first fret A# (Bb). This system is the same for all strings except the third string, which must be stopped four frets above the second string. The following examples show the system of unison notes as they shift up the fingerboard on to the next lower string.
The circled numbers (e.g., 2) indicate which string each note is to be played on. These string markings are common in music which does not contain tablature.