You will find that your left hand will have to pivot at the wrist to reach different notes and play different chords.? You must place the tips of your fingers on the strings without obstructing other strings and you will have to stretch your fingers across the fret board.? In some cases you will use your thumb to reach the 6th string.? What this means is that you do not grip the neck of your instrument tightly or in any one fixed position.? The neck is between your thumb and first finger with the thumb lightly supporting the back when necessary.? The thumb at the back of the neck gives you the support to press down with your fingers but will pivot according to where you place your fingers.? The thumb will normally be supporting the back of the neck at the position of the note you are playing but there are no strict rules to follow.?