I have tuned pianos for a living since the spring of 1977. In that time I have tuned, by my logbook, a little over eleven thousand of them, in living rooms, church basements, school gymnasiums, a bowling alley, two funeral homes and a barn. Almost every one of those visits included the question, asked politely while I had my head inside the instrument, "So what is it you're actually doing in there?" This page is the long answer. I have tried to make it accurate without making it a textbook, and to say plainly where something is my opinion.

The short answer, if you'd rather: a piano has about 230 strings pulled tight across a cast-iron frame, each one wound around a steel pin driven into a block of hard maple. I turn the pins, one at a time, with a lever, until each string sounds the right pitch, and "right" means right in relation to all the others. It takes about an hour and a half if the piano is in decent shape, and I listen the whole time. That's it. Everything below is the "why."

A tuning has to start from somewhere, and the somewhere is the A above middle C, which by international agreement vibrates 440 times per second. Every orchestra tunes to it; every tuning fork in my case is stamped with it. I carry a fork and, these days, an electronic device that gives the same note, and I check one against the other because forks are honest and electronics are not always.

Everything else in the piano is set from that one A. If the A is right and the rest of the piano is tuned correctly in relation to it, the piano is "at pitch," and it will agree with any other properly tuned instrument in the world. If a piano has sat for years and drifted a quarter-tone flat, and I simply tune it to itself, it will sound fine alone and wrong with the flute. So I bring it back to A440 first. See pitch raising, below.

Here is the part that surprises people. A piano is deliberately tuned a little bit out of tune, on purpose, everywhere, and it has to be.

Nature gives us perfectly pure intervals: when two strings vibrate in a ratio of exactly 3 to 2, they make a perfect fifth that is perfectly smooth, with no wavering. The trouble is that if you tune a piano by stacking up pure fifths, C to G to D to A and so on around all twelve notes, you do not arrive back at a C that agrees with the one you started from. You miss by a small but very audible amount. The pure intervals don't fit inside an octave. This was known centuries ago and has been argued about ever since.

Equal temperament is the compromise nearly every piano on earth is tuned to today. You divide the octave into twelve exactly equal half-steps, so that every key is equally usable and a piece can move from C major to F-sharp major without anything sounding strange. The cost is that no interval except the octave is perfectly pure. Every fifth is very slightly narrow, by an amount so small most people never notice; every major third is noticeably wide, and if you listen to a major third on a well-tuned piano you can hear it shimmer.

That shimmer is what I listen for. When two notes are nearly but not quite in a pure ratio, the sound wavers at a steady rate: it "beats." A tuner setting equal temperament isn't trying to eliminate the beats; he's setting each interval to beat at a particular, predictable speed, a little faster as you go up the keyboard. A fifth in the middle of the piano should beat slowly, about once every second or so; the major thirds beat much faster, and each one a little faster than the one below it. When the beats all progress evenly, the temperament is right. It is done entirely by ear, and it is the part of this trade that takes years.

The tools: a tuning lever (a steel handle with a socket that fits the square head of the tuning pin; some call it a hammer), a tuning fork, a set of mutes, which are little wedges of rubber and a long strip of felt, and my ears. On a grand I take off the music desk and the fallboard; on an upright I take off the front panels. Then, in order:

1. Mute the outer strings. Most notes in the middle and upper piano have three strings each, tuned in unison, struck by one hammer; the tenor has pairs of copper-wound strings, and the bottom notes have one thick wound string each. I weave the felt strip between the strings so that only one string of each note sounds. Now I can hear one note at a time.
2. Set the A from the fork. The A above middle C, to 440.
3. Set the temperament. In an octave in the middle of the piano, usually from the F below middle C to the F above, I tune the twelve notes one by one, using fourths and fifths from the notes already set, and checking each new note against the thirds and sixths it makes with the others. Each interval has to beat at the right speed. If a check interval beats too fast, something is wrong and I go back. This octave is the foundation; if it's wrong, the whole piano is wrong. It takes twenty to thirty minutes of careful listening.
4. Tune outward by octaves. With the middle octave set, I tune each note above it to the same note an octave below, and each note below it to the octave above, all the way to both ends, checking with fifths, tenths and other intervals as I go. Here's a detail: piano strings are stiff steel, and a stiff string's overtones run very slightly sharp of where the mathematics says they should. To make the octaves sound pure to the ear, the treble has to be tuned a hair sharp and the bass a hair flat, more so toward the extremes. This is called stretch, and every piano needs a different amount. Tuners argue about it the way cooks argue about salt.
5. Tune the unisons. Pull the felt strip out a little at a time and tune the second and third strings of each note to the first, so that all three sound as one string. A unison is done when the wavering stops entirely and the note goes clean. Bad unisons are what most people mean when they say a piano sounds "honky-tonk."
6. Set the pins. This is the part of the craft that isn't about hearing. A tuning pin that's simply turned to pitch will slip back when the piano is played hard. So each string is tuned slightly sharp, then eased back down so the pin and the string are settled and stable. It's a feel in the hand, and it's the difference between a tuning that lasts and one that's gone in a month. When I test a tuning I pound the keys, hard. If the pitch holds, it's set.

Then I put the panels back, play something, and take the check. About ninety minutes on a well-kept piano, longer on one that's neglected. I do it by ear; I keep the electronic device for checking the A and for pitch raises, and I'm not against the good ones, but the ear hears the piano in front of it and the machine hears numbers.

The main reason is not playing, and it is not moving (though both matter). The main reason is humidity, and the part of the piano that responds to it is the soundboard: the big, thin sheet of spruce behind the strings that turns their vibration into sound you can hear. Wood takes on moisture from the air and gives it up again. When the air is humid, the soundboard swells and its slight arch (the "crown") rises; that pushes the bridges the strings run over a little higher, which tightens the strings, which raises the pitch, mostly in the middle of the piano. When the air dries out, the board shrinks and flattens, the strings slacken, and the pitch drops. The change is not even across the keyboard, which is why a piano doesn't just go uniformly flat; it goes out of tune with itself.

In this part of the country the humid months are June through September and the dry months are when the furnace runs, November through March. A piano tuned in July and left alone will be noticeably out by January and back somewhere close to where it started by the next July, having spent the whole year out of tune in between. That is the seasonal cycle, and every piano that lives in a house with seasons goes through it.

The other causes, smaller but real: new strings stretching (a new piano or one that's just been restrung needs tuning several times its first year); hard playing, which drives the pitch of the notes that get hit the most; moving, which is less about the truck than about the new house having different air; time, since pins loosen and pinblocks age; and sudden changes, like the piano that sits beside a heat register, in front of a window, or against an outside wall.

What you can do about it: keep the piano away from heat registers, radiators, fireplaces and direct sun; keep it off outside walls if you can; and keep the room's humidity as steady as your house allows, with a humidifier in the winter if the air gets very dry. There are humidity control systems that mount inside the piano, and for a good instrument in a difficult room I recommend them. I will not sell you one; ask your technician.

Twice a year, for a piano that is played and cared about: once after the heat goes on and the house has dried out, once after it goes off and the humid weather has settled in. That catches the piano at each end of its seasonal swing. A piano in a concert hall or a teaching studio gets tuned far more often; a piano that's furniture gets tuned when the grandchildren visit, and I tune those too, but the instrument is not well served by it. Once a year is the least I'd suggest for anything you want to keep.

Pitch raising. If a piano hasn't been tuned in some years it will usually have drifted well flat of A440, sometimes a quarter tone, sometimes more. You can't simply tune such a piano to pitch in one pass: as you tighten each string the added tension on the frame and soundboard pulls the strings you've already done back down again. So the technician makes a fast, rough pass first, bringing everything up to slightly above pitch and letting the whole structure settle, and then does the fine tuning on top of it. Sometimes it takes two rough passes. It's more work, it's usually a little more money, and the piano won't hold as well as one tuned regularly until it's been tuned a few times more. That's not a sales pitch; it's the physics of twenty tons of string tension finding a new balance.

People call me and say "the piano needs tuning, some of the keys stick." Sticking keys are not a tuning problem. There are three different kinds of work on a piano and it helps to know which one you need.

• Tuning is the pitch of the strings. That is all it is. Everything on this page above this point.
• Regulation is the adjustment of the action: the several thousand wooden, felt and metal parts between your finger and the string. Each key works a lever that throws a felt hammer at the string and catches it as it falls back. Every one of those parts has an adjustment: how far the key goes down, how high the hammer rests, how close it gets to the string before it lets go (the "let-off"), how quickly it's ready to strike again, how the damper lifts. Felt wears and compresses, wood shifts, and over years the action gets uneven: some notes heavy, some light, some that won't repeat fast, some that don't sound at all on a soft touch. Regulation puts all of it back to specification. On a piano that's played seriously, a full regulation every several years; touch-ups at tuning time in between. Sticking keys, by the way, are usually swollen wood or a bit of something (a pencil, a crayon, a paper clip; see stories) down between the keys, and I fix those while I'm there.
• Voicing is the tone. The hammers are dense felt, and over years of hitting steel strings the felt gets packed hard and grooved, and the tone gets bright and harsh, or, on a piano that's had the hammers reshaped too often, dull. Voicing means reshaping the hammers and needling the felt to soften it, or hardening it with a treatment, to even out the tone from note to note and make the piano sound the way it should. It's separate work, it's slow, and it makes a bigger difference to how a piano sounds than most people expect.

When you call a technician, say what you're hearing or feeling ("some notes sound sour," "the keys feel heavy," "it sounds tinny") rather than diagnosing it. We can tell which of the three it is in about a minute at the keyboard.

A good used piano is one of the best bargains there is, and a bad one is a large, heavy way to lose several hundred dollars and your child's interest in music. Here's what I look at when a customer asks me to check one out, and what you can check yourself before you call me.

1. Play every key, softly and hard. Every note should sound; every key should come back up promptly; nothing should click, buzz or rattle. A few dead or sticky notes are usually fixable; a whole section is a warning.
2. Listen to whether it's in tune with itself. Play octaves up and down. If it's mildly out, that's normal for a piano that's been sitting. If it's a mess, ask when it was last tuned. If the answer is "it doesn't need it, it was tuned when we bought it" and they bought it in 1971, expect a pitch raise and possibly worse.
3. Look inside. Take the front off an upright (the top lid lifts and the front board usually lifts out) or look under the lid of a grand. You're looking for rust on the strings and tuning pins, cracks in the soundboard (a crack isn't necessarily fatal, but a crack that buzzes is a repair), and cracks in the bridges where the strings cross, which are serious. Look at the hammers: deep string grooves in the felt mean a lot of use. Check for mouse droppings, moth damage in the felts, and anything that looks like it's been wet.
4. The tuning pins must be tight in the pinblock, or the piano will not hold a tuning and there is no cheap fix. I check this with my lever; you can't, but you can ask when it was last tuned and whether it held. A piano that "goes out right after it's tuned" has loose pins or a cracked block, and I would walk away.
5. Size and type. Uprights are measured by height: a spinet is the shortest (under about 40 inches), and it has a "drop action" that sits below the keys and is awkward to service; the tone is small. A console (40 to 44 inches or so) has a direct action and is a fine home piano. A studio (45 to 48) is what schools buy; sturdy and full-toned. An old full upright (50 inches and more) from before about 1930 can be a wonderful instrument or a wreck, depending on how it's lived. Grands: the longer the string, the better the bass, all else equal; a baby grand under five feet has bass strings shorter than a good upright's. Don't buy a grand for the shape.
6. Age. The serial number, usually stamped on the iron plate near the tuning pins, dates the piano; a technician or a library reference will tell you the year. A well-made piano properly cared for is good for fifty years and more; a cheaply made one is not, whatever its age. Brand names mean something, but condition means more.
7. Where it lived. A piano that spent a decade in an unheated garage, a damp basement or a sunroom has been through more than one that sat in a living room for fifty years. Ask.
8. Get it looked at. For anything over a few hundred dollars, pay a technician for an hour to inspect it before you buy. I charge for this what I charge for a tuning, and I have talked more people out of pianos than into them. Then budget for the move (hire piano movers, not your brother-in-law) and for a tuning about two weeks after it's in the house and has settled.

Free pianos: some are treasures and most are free for a reason. All of the above applies, and be twice as careful.

The mouse. A console in a farmhouse, 1983, "sounds funny in the middle." Took the panel off and a family of mice had built a nest of shredded hymnal pages against the soundboard behind the tenor strings, warm and dry and next to the hammers. The mother was in residence. I put the panel back on, and the lady of the house and I had a conversation about whose job the next step was. (Hers. She was very capable about it.) The piano tuned up fine afterward, once the felts were replaced. It still smelled faintly of mouse for a year, she told me. There was nothing to be done about that.

The thirty-year tuning. Nearly every technician has this one. "It was tuned when we bought it," she said, "so it shouldn't need much." They bought it in 1966. It was nearly a half-step flat, evenly, which means it had drifted down a little each year and settled into a kind of peace with itself; it sounded lovely, all alone, in the wrong key. Two rough passes and a fine tuning, three hours, and she said, sincerely, "it sounds so bright now, is that normal?" It was not bright. It was at pitch. She had never heard it at pitch.

The pennies. A boy of about seven, an upright in the den, and a mother who could not understand why the keys from middle C up to about G would not go down. Fourteen pennies, two nickels, a hair barrette and, deep in the bass section, a plastic dinosaur, all dropped into the gap behind the fallboard one at a time over what must have been months. The boy, when asked, said he was "putting them in the bank." I gave him the dinosaur back. The mother kept the money.

The church. An old upright in a country church, tuned once a year by me since 1979, and before that by a man who tuned it to itself for decades because there was nothing else there to agree with. When I first brought it up to pitch, the organist, a woman then in her seventies who had played it since girlhood, sat down, played a hymn, stopped, and said, "That isn't right." It was the first time in her life it had been right. She played the whole service that Sunday in the key she'd always played it in and complained to the pastor. By Christmas she had come around. She played it until 1994 and I still tune it, and her daughter plays it now.

The barn. A grand, a good one, in a barn, under a tarp, for the summer while a house was renovated. The customer called in September and said it "sounded a little off." The soundboard had taken on so much moisture it was nearly a quarter tone sharp in the middle and the action had swollen so that half the keys stuck. It came back, mostly, over a year and four tunings after it was in the finished house. I have said "not the barn" to a number of people since. Nobody thinks it applies to them.

The best one. A studio upright in a school music room, tuned every August before the year began, twenty years. One August a young woman waited in the hall until I finished, then came in and said she had learned to play on that piano, had gone on to a conservatory, and was teaching now, and she had wanted to tell the man who kept it in tune. That was all. I have thought about it on a good many Tuesdays since.

• Carter Piano Studio — Mrs. Carter teaches on two pianos I have tuned for years. She sends me students' parents, and I send her their children.
• The Community String Quartet — four amateurs and the problem of intonation, which for string players is a problem every note, not twice a year.
• The Amateur Cellist — Helen W.
• Opera for Beginners — Gerald F., retired music teacher.
• Clockwork: Repairing Old Clocks — Mr. Kaminski's trade is a cousin of mine: old mechanisms, felt and brass, patience.