There are three 'half step cluster' positions on the fretboard between the open strings and the 10th fret that correspond to middle B - C - E - F on the piano keyboard. These are shown in gray in the diagram above. The 'semi clusters' E and F below middle C are shown in blue, and B - C above middle C are shown in pink.
The diagram only names the notes of the four top strings so as to emphasize the symmetry within this string group.
If you have a piano, compare the pitches as shown in the diagram.
You can see that moving from lower string/lower fret to higher string/higher fret will quickly get you to higher pitches, while moving from higher string/lower fret to lower string/higher fret will enable you to change fret positions while essentially staying within the same tonal range.
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