Because each fret on the fingerboard marks the distance of a semitone, wherever a string is played at any fret, the next fret 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 playing the sixth string at the fifth fret, we obtain the A which is a unison of the open fifth string A. Similarly, if we play 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 played 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.
Note: refer to the section, "Tuning the Guitar to Itself" at the end of this, book.