A new string often leaves the saddle and nut in a soft arc. Until it bends into a crisp angle, the string can shift slightly as you play and bend, and the effective string length isn't exact.
How to set witness points:
- Install the string and tune it close to pitch.
- Place your thumb on the string just on the tailpiece side of the saddle.
- Press down firmly so the string bends sharply over the saddle.
- Do the same just behind the nut, on the headstock side.
- Retune. The pitch will have dropped slightly.
Do this for each string, then gently settle the strings by lifting them a little along their length and retuning a few times. Combined, these steps help a fresh set hold pitch much sooner.
On bass, witness points matter even more, because heavy strings resist bending over the saddle. Press firmly, but don't pinch the string with pliers or bend it so hard that you kink the winding.