'Nut' Adjustment so that your open strings match your fingered notes...
...The 'Nut' is the small ebony wood block at the peg box end of the fingerboard. It supports the end of the vibrating string and the string groove is shaped in such a way that it allows the strings to 'fall away' toward the peg box and hold each string at the correct height over the fingerboard. The wood grain goes in the direction of across the instrument. The ebony wood is strong enough to keep the string from wearing a groove in the wood.
...The vibrating string interacts with the nut and then its' vibration is reflected back up the string. It is important that the frequency of the wood where the string contacts it is the same as the frequency of the fingerboard under that same string. That way the open strings will have the same strength as the fingered string notes. Both of these should be also matched exactly by the tail piece frequency just where the string first contacts it . When these three conditions exist, then the maximum power and clarity will be developed by the instrument.
...The nut needs to be adjusted at each string position. Adjustment is very easy. The necessary tools are a small sanding stick or a small flat jewler's file and some masking tape to protect the adjacent varnish (use little pressure when applying the tape or the varnish might be lifted when the tape is removed.
...If the downward tap (tap tone) at the string contact point is too low, then sand or file some material from the nearest end of the nut until it matches.
...If the tap is too high, then file or sand some material from the 'shoulder' shape on the peg-box side of the nut in line with that string. (As the ebony wood is sloping away from the string contact area toward the peg box..)
...In listening to the tap tone, identify for the lowest overtone in the tap sound and then use that tone to deside how to adjust.
Remove material here (behind the string you want to adjust) to lower the frequency at the string contact point.
String contact point.
Fingerboard in side profile