1. New strings are still stretching
This is the most common cause. New strings stretch, so pitch drops steadily for a while after fitting.
Fix: after stringing, gently lift each string a few times along its length (do not yank), tune up, play for a few minutes, then tune again. Two or three rounds and it usually settles within a day.
2. The string is slipping on the tuning post
If a string is not wound enough times, or the windings overlap, it can slip slightly while you play and the pitch drifts.
Fix: leave about two fingers of slack, lock the first winding under the string end, then wind the rest neatly below it. Two or three turns for steel strings, three or four for nylon.
3. Temperature and humidity swings
Wood expands and contracts with humidity, and a moving neck or top changes string tension. Air conditioning, winter heating and moving indoors from outside all shift pitch noticeably.
Fix: keep the instrument where humidity is fairly stable (45-55% is ideal). Avoid windows, radiators and leaving it in a car. This is normal behaviour, not a fault.
4. Loose tuners or a pinching nut
A loose tuner screw, worn gears, or a nut slot that is too narrow and grips the string all produce the classic tune-and-it-drops problem.
Fix: check the screws on the tuner buttons first. If a tuner turns with obvious play or slips, it may need replacing. For a gripping nut, have the slot filed by a technician, or add a little pencil graphite for lubrication.
5. Neck and bridge movement
A forward-bowing neck or a lifting bridge (common on inexpensive acoustics) keeps the whole instrument unstable.
Fix: look for a visibly curved neck or a gap between the bridge and the top. Do not attempt these repairs yourself; take the instrument to a professional.
6. Picking too hard
Hitting a string hard pushes its pitch up momentarily, especially on the low strings. If you tune gently and then strum hard, it will sound out of tune.
Fix: tune with the same attack you actually play with, so the tuned pitch matches real playing.
A three-minute check before practice
1) Strum the open strings and listen for the obvious offender. 2) Open the online tuner and check string by string, starting from the 6th. 3) Pay attention to the string that drifts most; it usually reveals the problem early. 4) Play through once more after tuning, because adjusting one string slightly affects the others.
If the same string always drifts far more than the rest, the winding or the tuner on that string is the likely culprit — deal with it first.
