# A pitch log records the room, not the piano

> A piano tuner's pitch log isn't really about the piano. It's a year of a room's humidity, recorded one cents deviation at a time, note by note.

August 19, 2026 · https://justog.club/stories/piano-tuner-pitch-log

A440 at the head, large. Twelve rows beneath it, C through B, a cents-deviation figure pencilled
beside each note. Three are circled. A note in red: room too dry.

Nothing here measures the instrument. Everything here measures the weather.

## What is a pitch log, and why does it really describe the room?

A row of twelve notes, each with a cents-deviation figure showing how far it's drifted from where
it was last set.

A piano goes out of tune because its wooden soundboard swells and shrinks as humidity in the room
changes, pulling every string with it. The tuner isn't really measuring the piano; they're measuring
what a year of weather did to a large wooden instrument that happens to have strings and a keyboard
attached. Log the temperature and humidity alongside the notes, as this one does — 23.6°C, 28% RH —
and the whole page reads less like music and more like a weather station report.

## Why does A read exactly 0.0 while every other note gets a number?

Because A440 is the reference the rest of the log is measured against, not a note that happened to
come out perfect.

Every other figure on the page is a distance from that one point — D sharp two point six cents
sharp, A sharp five point six cents flat — and none of those numbers would mean anything without a
fixed zero to measure from. Setting A to 0.0 first is what makes the rest of the log legible as a
single set of readings instead of twelve unrelated facts.

## Which readings are worth circling on the log?

The outliers, not the whole row — the notes that moved enough to actually need attention.

A log where every note drifted by roughly the same amount says the room did something uniform:
humidity changed, the whole soundboard responded together. A log where two or three notes are
circled well outside that pattern says something more local happened — a string, not the room — and
that's the distinction the circles exist to draw. Three circled against nine plain is more useful,
and more honest, than circling nothing or circling all twelve.

## Can a real tuner's pitch log go on a shirt?

The method is nobody's to license; the readings from a real piano, in a real room, belong to that
piano's owner and the tuner who logged them.

Cents deviation, A440 as reference, temperature and humidity beside the notes — that's how the
trade records a tuning, and a fact about how a trade works isn't property. A named
concert hall's actual log, or a particular piano's service history, is a different matter, in the
same way [a real service card belongs to a specific watch rather than to the
format](/stories/watchmaker-service-card). Invented figures on a real method get the whole
thing right without asking anyone's permission.

## Does this work for an instrument that isn't a piano?

Anything tuned by ear against a fixed reference, rather than built to a fixed pitch and left alone.

[A choir's four numbers work from the same reference-note logic](/stories/choir-and-orchestra-shirts):
everyone tunes against the same A, and everything else is a distance from it. A harpsichord, a pipe
organ, a set of tuned percussion — anything that drifts and gets checked periodically has a version
of this log, though few instruments make the room's humidity quite as legible as a piano's
soundboard does.

## Isn't a log about drift a strange thing to be proud of?

No — drift isn't a failure here, it's the whole reason the job exists.

An instrument that stayed put wouldn't need a tuner at all, and the log isn't evidence of a
problem; it's evidence that somebody checked. The circled notes aren't a complaint about the piano.
They're the parts of the room's story that were worth writing down in red instead of pencil.

## Does a pencil log print well?

Yes, if the pencil stays pencil-grey rather than trying to look photographed.

The dense figures want more room than a real page gives them — twelve rows of small cents values
will fill in at shirt scale unless they're set larger and spaced further apart than the tuner
actually wrote them, because [fine pencil detail is exactly the kind of mark that survives on paper
and not on fabric](/stories/bold-shapes-survive-fabric). Charcoal on bone carries the log
itself; the single red is worth keeping to the circled figures and the margin note, so it stays a
flag rather than a second competing colour.

## How do I get one made?

Describe the log at [JustOG](/) — which notes moved, the room conditions, and what the margin note
says. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to
order and shipped.

Designs other people have published are in the [shop](/shop).

The piano was in tune in April. By August the room had an opinion about that.