A tuner measures in cents, which are hundredths of a semitone, and a piano in a stable room drifts a few cents over months. On this one, F below middle C goes flat by six cents a week, and nothing else on the instrument moves at all.
I have tuned pianos for thirty one years and I do about forty schools.
The units, because the whole thing hangs on them.
A semitone is divided into a hundred cents. A good ear notices two or three cents on a sustained note and almost nobody notices one. A piano that has been moved, or that has gone through a heating season, will be ten or fifteen cents flat overall, and that is a normal tuning.
Drift is not random. It is the soundboard and the frame responding to humidity, so it happens to the whole instrument together, and the middle goes first because that is where the tension is highest.
I carry an electronic tuning device as well as my ears, because a school wants a report and because a figure settles arguments.
The piano in this hall is an upright from about 1968, overstrung, iron frame, and it lives against the inside wall of a hall with no radiator within eight metres of it.
It holds its tuning better than most school pianos because nobody plays it hard.
F below middle C goes flat at six cents a week.
That note. Not the F above. Not the E or the F sharp either side of it. One note, three strings in its unison, and all three go flat together and stay in tune with each other.
I have measured it on eleven visits over four years, with two different devices and once with a borrowed strobe tuner that cost more than my van.
Six cents, plus or minus one, every week since the last time I touched it.
Why that should not happen.
A single note going flat is the commonest fault in the trade. Almost always it is a loose tuning pin in the pin block, and you can feel it the moment you put the lever on: the pin turns smoothly instead of gripping, and a loose pin slips in minutes, not evenly over weeks.
Those three pins are tight. I have measured the torque with a gauge, which is not standard practice and which I did because I was losing the argument with myself. They need about sixty inch pounds to move, which is a healthy pin block.
A string stretching. New strings creep and settle and are flat for a few weeks, and these are not new strings. I replaced all three in 2022 to see what would happen, and the new ones settled in about three weeks, as they should, and then went on going flat at six cents a week.
The bridge, the soundboard, the agraffe, the hitch pin. I have had the action out, the back off, a torch and a mirror and a straight edge on all of it. The soundboard has no crack near that note. The bridge pins are sound and the bearing is right.
A heat source behind the wall. The obvious thing, because local heat would slacken one part of the instrument. There is a cupboard on the other side of that wall with nothing in it but chairs, and I have had a thermometer in it and on the wall itself. The wall is the temperature of the hall, and the back of the piano is the temperature of the hall, and I have a week of logged readings from a little data logger I leave in the bottom of the case.
What I do about it.
I tune it. Every visit, and the school pays a standard tuning rate twice a year, and I have stopped charging for the note.
That is the arrangement I have with the music teacher. If I am within fifteen miles on a Thursday I call in and do that one note, and it takes four minutes, and she gives me a coffee and a chocolate biscuit and we talk about her son's driving test.
She asked me once whether it mattered. It does and it does not. Six cents a week means that by the end of a half term it is forty cents flat, which is most of a quarter tone, and a child playing a scale hears that something is wrong with the piano and not with their finger, and the ones who are any good stop trusting the instrument.
So no, it is not dangerous and it is not expensive, and yes, it is the difference between a room where a child learns and a room where they do not.
The part I have not told her.
Two years ago I started writing the reading in my book before I tuned it, to see whether the rate was changing.
It is not. It is six cents a week and it has been since 2021.
What changed is what I found when I went back over my book to check that.
I tuned that piano for the first time in 2004, for the woman before her, and my notes from then and through about 2015 say nothing about an F. I am meticulous about the book and a note that went flat at that rate would be in it every single visit.
The first entry is October 2019, in my own hand, and it says F28 flat 6 cents, recheck.
And underneath it, in the same pen, in my own writing, it says in the hall on my own, which is not a thing I write, and which I have no memory of writing, and which is true of every visit I have made to that hall in twenty one years.