How a Piano Works: Hammers, Strings, Soundboard, and Pedals

By Tissimo Team

Published November 6, 2026

Press a key on the piano and you set off a chain of about twenty mechanical parts that ends with a felt hammer striking a string and bouncing away before the string can even begin to vibrate. Everything else — the sound you hear, the way it fades, the way the pedals change it — follows from that single mechanical event.

From key to string: the action

The piano action is a lever system between the key and the strings. When you press a key at the front, the far end rises and pushes up a part called the wippen. The wippen carries a small pivoting lever called the jack (or escapement jack). As the wippen rises, the jack pushes the hammer roller upward, launching the hammer toward the string.

Diagram of the piano action: a key pivots at a balance rail; the wippen rises at its far end; the jack pushes a hammer roller upward; the hammer, covered in felt, rises toward the string; a damper rests on the string above

The key-to-string action chain. Pressing the key at the front raises the wippen; the jack on the wippen pushes the hammer roller, launching the hammer toward the string. The damper rests on the string until the key lifts it.

Just before the hammer reaches the string, the toe of the jack strikes a small regulating (let-off) button, which forces the jack to pivot out from under the hammer roller. The hammer is now in free flight — no longer connected to anything. It strikes the string and rebounds immediately, so the string vibrates freely and is not muffled by the hammer sitting on it. This pivot-and-release is the escapement, and it is the defining mechanism of the piano. Without it, the hammer would rest against the string and kill the sound.

The escapement also makes it possible to repeat a note quickly, because the jack can reset while the key is still partly held down. The modern repetition mechanism (Érard's patent, 1821) adds a repetition lever that catches the hammer partway back, so the jack can re-engage without the key returning all the way.

The hammer

Piano hammers are compressed felt stretched over a wooden core. The felt is denser in the outer layers than the inner ones, which lets the hammer compress slightly on impact and produce a warmer tone at soft dynamics and a brighter, harder tone when struck firmly. Over time the felt develops grooves where it contacts the string; technicians periodically needle or reshape the felt to restore tone.

The hammers for the lowest bass notes can be more than twice the size of those for the highest treble notes, because the low strings are thicker and need more energy to vibrate efficiently.

The speed at which the hammer travels when it strikes — determined by how fast and how firmly you press the key — controls the volume and to some extent the tone. A slowly-moving hammer compresses the felt gently and produces a soft, round tone; a fast-moving hammer compresses it more and produces a louder, brighter tone. This is why the piano's full name is pianoforte (Italian: "soft–loud"): unlike the harpsichord, which plucks its strings at a fixed force regardless of touch, the piano responds to the player's physical pressure. The velocity of the hammer at the moment of string contact is the instrument's primary expressive variable.

The damper

Resting on most of the strings, above the hammers, is a row of dampers — small blocks with felt pads that press against the strings. When you press a key, the wippen mechanism also raises the damper for that note, freeing its strings to vibrate. When you release the key, the damper drops back and silences the note.

The damper resting on its strings (key up) and lifted (key held). The felt pad silences the strings until the key is pressed. When the key is held down, the damper stays lifted and the strings vibrate freely. Releasing the key drops the damper back onto the strings and stops the sound.

The upper 18 or so notes of the piano have no dampers, because those short strings lose energy quickly on their own and do not need them. This is why very high notes continue to ring briefly even without the pedal. Everything below that range — essentially the whole piano except the last octave and a half — is damped on key release.

The strings: pitch, length, tension, and mass

A stretched string vibrates at a frequency determined by three quantities: its length, its tension, and its mass per unit length (essentially how thick it is). The relationship is given by Mersenne's law:

Diagram showing three strings side by side: one long and thick (low pitch), one medium (middle pitch), one short and thin (high pitch); below each, a frequency label showing that shorter/lighter/tighter strings vibrate faster

How string length, tension, and mass per unit length determine pitch. Shorter strings vibrate faster (higher pitch); less mass per unit length means faster vibration; greater tension raises pitch. Piano makers adjust all three across the range.

Shorter strings vibrate faster → higher pitch.
Thinner strings (less mass per unit length) vibrate faster → higher pitch.
Higher tension causes faster vibration → higher pitch.

Piano strings are strung at very high tension — a modern concert grand has about 220 strings with a combined pull of roughly 20 tonnes — so the instrument needs an extremely rigid frame to hold them. That frame is a cast-iron plate (also called the harp or frame), which is bolted to the wooden case and carries essentially all of the string tension. The plate is typically painted gold and is the most visible internal component when you open the lid. Before the development of the full cast-iron plate in the mid-nineteenth century, piano frames were made of wood reinforced with iron bars; they could not hold enough tension to give the instrument the power and sustain expected of concert instruments today. The all-iron frame, perfected by Steinway and others in the 1850s and 1860s, made the modern concert grand possible.

Diagram of a grand piano interior seen from above: the cast-iron plate occupies most of the visible area; tuning pins are shown in a row along the top; strings run from the tuning pins over the bridge on the soundboard below the plate; the bridge position is indicated on the visible soundboard area

The cast-iron plate (harp) of a grand piano, viewed from above with the lid open. Tuning pins hold the top ends of the strings; the strings run across the plate and over the bridge to the hitch pins at the far end. The plate carries nearly all of the tension, leaving the wooden case free to resonate.

Bass strings: single and wound

In the bass, single strings would need to be impractically long to vibrate at the required low frequencies. Instead, the strings are copper-wound: a thin steel core wire is wound with a copper spiral, increasing the mass per unit length without making the string longer. Bass strings are always single — one string per note — and visibly thicker and longer than treble strings.

Diagram comparing two string cross-sections side by side: on the left, a plain steel treble string (small circular cross-section, uniform); on the right, a copper-wound bass string (a thin steel core surrounded by a tightly coiled copper helix, making a larger diameter); length labels show the bass string is longer

Cross-sections of a plain treble string (left) and a copper-wound bass string (right). The copper winding dramatically increases the mass per unit length, allowing a low-pitched note to fit within the instrument's physical length without the string becoming impossibly thick steel.

Treble strings: bichords and trichords

Middle and upper registers use multiple strings per note. Most middle notes use two strings tuned to the same pitch (a bichord), and the upper two-thirds of the keyboard typically use three strings (a trichord). Multiple strings serve two purposes: they increase volume (more string surface area radiating sound) and they allow the note to sustain longer, because the strings share the energy and each loses it more slowly than a single string would.

Diagram of three groups of strings as seen from above: a single thick wound string in the bass; two parallel thin strings in the mid range labeled bichord; three parallel thin strings in the treble labeled trichord; note names and octave positions labeled below

String groupings across the range. Bass notes use a single copper-wound string. Middle notes use two strings in unison (bichord). Upper notes use three strings in unison (trichord). All strings for one note are tuned to exactly the same pitch and struck together by one hammer.

All the strings for a single note are tuned to the same pitch and struck together by one hammer, whose width is sized to contact all of them at once.

The soundboard and bridge

A string vibrating on its own is nearly inaudible — its thin diameter moves too little air. The energy of the vibration is transferred via the bridge (a strip of hard maple glued to the top surface of the soundboard) to the soundboard itself.

The soundboard is a large, lightly arched panel of quarter-sawn spruce, typically 8–10 mm thick. Spruce is chosen for its exceptional stiffness-to-weight ratio: it moves easily in response to the bridge's driving force yet is stiff enough to resist deformation. Because it has a large surface area, the soundboard moves far more air than the string alone and amplifies the sound dramatically.

Diagram showing a string under tension; an arrow runs from the string down through the bridge to the soundboard; ripple lines around the soundboard indicate sound radiation in all directions

The vibration path. The string vibrates and transmits energy via the bridge to the spruce soundboard. The soundboard's large surface area moves air efficiently, producing the audible sound.

The soundboard also shapes the tone: the particular resonances of a soundboard at different frequencies contribute to the characteristic color of a given instrument. This is why two concert grands of the same model, made in the same factory, can sound perceptibly different from each other. The slight arch of the soundboard — called the crown — is built in during manufacture by bending and gluing the panel under pressure. Over decades, the crown can flatten as the wood relaxes, contributing to the gradual change in tone that aging instruments develop. The ribs glued across the underside of the soundboard control how the crown is distributed and how vibration spreads across the panel's width.

Tuning: equal temperament

A modern piano is tuned to equal temperament: the octave is divided into 12 equal semitones, each a frequency ratio of exactly 2^(1/12) ≈ 1.0595. This means every interval except the octave is a slight compromise — a perfect fifth in equal temperament is about 2 cents narrower than a pure 3:2 ratio — but the advantage is that the instrument sounds equally in tune in every key, which was essential once music began modulating freely between keys. The harmonic series explains why pure intervals are simpler frequency ratios and why equal temperament approximates but does not exactly reproduce them.

Piano tuning is done by adjusting each string's tension with a tuning hammer at the tuning pin. Because the high strings are strung at extreme tension, their actual vibrating frequency is raised slightly above the equal-tempered value by inharmonicity — the stiffness of the string makes its upper partials slightly sharp. Skilled piano tuners compensate by stretching the octaves slightly: the top of the instrument is tuned slightly higher than pure, and the bass slightly lower, so the partials of bass notes align with the fundamental of treble notes.

Diagram labeled Tuning stretch: a horizontal axis representing the piano keyboard from bass to treble; a straight line labeled pure equal temperament runs flat across the middle; a curved line labeled actual piano tuning dips slightly below in the bass and rises slightly above in the treble, crossing the pure line near middle C; the vertical axis is labeled deviation in cents

Octave stretch in piano tuning. A perfectly equal-tempered tuning (straight line) would cause the high partials of bass strings and the fundamentals of treble strings to beat against each other, because inharmonicity sharpens the partials. Tuners compensate by tuning the bass slightly flat of pure and the treble slightly sharp — a curve that follows the instrument's own inharmonicity.

Grand vs upright: two ways to return the hammer

In a grand piano, the hammers hang horizontally from a rail above the strings (the strings run horizontally, away from the player). After a hammer strikes, gravity pulls it back toward its rest position. This is fast and reliable, and it allows the repetition mechanism to work smoothly.

Diagram comparing the grand action (hammer horizontal, gravity return, strings horizontal) with the upright action (hammer vertical, spring return, strings vertical); arrows show the direction of hammer travel and return in each case

Grand action (left): hammers strike upward or forward; gravity returns them. Upright action (right): hammers strike horizontally; a return spring pulls them back. The grand's gravity return is faster and allows the repetition lever to work efficiently at high speeds.

In an upright piano, the strings run vertically and the hammers strike horizontally. Gravity cannot return a horizontally traveling hammer; instead, a return spring (also called a bridle tape and spring assembly) pulls each hammer back after the stroke. The spring return is reliable but inherently slower than gravity, which is why upright actions cannot repeat notes as quickly as good grand actions. For most home practice this makes no practical difference, but fast repeated notes (such as the repeated notes in Ravel's Alborada del gracioso) are easier to control on a grand.

The three pedals

A modern grand piano has three pedals. Uprights sometimes have two, sometimes three, but the functions of the middle pedal often differ.

Diagram of three pedals viewed from the front: left labeled una corda / soft pedal, middle labeled sostenuto, right labeled sustain / damper pedal; a brief caption under each explains its function

The three pedals of a grand piano. Right: the sustain pedal, which lifts all dampers. Middle: the sostenuto, which sustains only the notes held when it is pressed. Left: the soft pedal (una corda), which shifts the action sideways.

The sustain pedal (right)

The sustain pedal (also called the damper pedal or loud pedal) lifts all the dampers at once. While it is held down, every note you play continues to ring after you release the key, and the undamped strings of the whole piano can vibrate sympathetically with the notes being played. This sympathetic resonance — undamped strings vibrating in response to energy they share with the notes being struck — is the source of the characteristic bloom and warmth that the sustain pedal adds to the sound. The technique, timing, and notation of the sustain pedal are covered in a separate guide on the Tissimo blog.

The soft pedal (left): una corda on a grand

On a grand piano, the left pedal is the una corda pedal. Pressing it shifts the entire action assembly a few millimetres to the right, so that in the treble each hammer (which is sized to strike all three strings of a trichord) strikes only two of its three strings. The hammer also contacts a part of the felt that is less grooved and softer than the central striking area, changing the timbre as well as the volume. The combined effect is a quieter, slightly veiled tone that is different in quality from simply playing softly without the pedal.

The term una corda (Italian: "one string") dates from early fortepianos, some of which shifted far enough that the hammer struck only one string. On modern grands, with triple-strung notes throughout most of the range, the shift typically produces a due corde ("two strings") effect in the treble rather than a true one-string contact. The pedal name has persisted, however.

Diagram showing the una corda shift on a grand: left, a hammer aligned with three strings; right, the same hammer shifted so it contacts only two strings; the third string is shaded to indicate it is not struck

The una corda pedal on a grand piano shifts the action sideways so each hammer contacts one fewer string. In the treble, where notes have three strings, this produces a due corde (two-string) effect.

On an upright piano, the left pedal works differently: it moves the hammers forward, closer to the strings, reducing the distance the hammer travels before impact and therefore limiting its striking force. This softens the tone but does not change the timbre in the same way as the una corda shift on a grand.

The sostenuto pedal (middle, grand)

The sostenuto pedal is the most selective of the three. When you hold down one or more keys and then press the sostenuto, a bar rises inside the piano and catches the dampers of only those notes — the ones that were raised when you pressed the pedal. Those dampers stay up while the pedal is held, allowing those specific notes to sustain. All other notes play and damp normally.

This makes it possible to hold a single bass note or chord in the pedal while playing other material with clean damping above it — an effect that is impossible to achieve with the sustain pedal, which lifts every damper equally. Ravel's Jeux d'eau and several Debussy preludes use the sostenuto to hold pedal tones while harmonies shift above them; on instruments without a true sostenuto it is common to use half-pedaling or careful re-pedaling as an approximation.

Diagram of the sostenuto mechanism: several dampers shown in a row; two are caught on a horizontal sostenuto bar (shaded), remaining held up while the others have returned to rest on their strings

The sostenuto pedal catches only the dampers that were already raised when the pedal was pressed. The shaded dampers stay up, sustaining those notes; all other dampers function normally.

The sostenuto is not universal on upright pianos. On many uprights — particularly those designed for home use — the middle pedal is instead a practice pedal (also called a muffler pedal): it lowers a strip of felt between all the hammers and all the strings, dramatically reducing the volume to a soft thud. This allows practice without disturbing others but changes the tone so much that it is unsuitable for musical use. Some European uprights have only two pedals and omit the middle entirely.

Putting it together: how a note sounds

When you press a key, the lever system raises the wippen, the jack launches the hammer, and the damper for that note rises. The hammer travels in free flight (escapement already disengaged), strikes one or more strings, and rebounds. The strings vibrate at the pitch set by their length, tension, and mass, and they transfer that vibration through the bridge to the soundboard, which radiates sound into the room. As long as you hold the key, the damper stays up and the note sustains. Release the key, and the damper drops back, stopping the vibration.

The sustain pedal adds one more layer: it keeps all dampers lifted independently of the keys, so the room fills with sympathetic resonance, and the sound lingers long after the keys are released. The soft pedal colors the tone at its source, by changing the hammer-to-string contact point. The sostenuto pedal selectively freezes a chosen set of bass tones while everything above it moves freely.

That chain — lever, escapement, free-flight hammer, string, bridge, soundboard — is essentially unchanged from Bartolomeo Cristofori's first instruments of around 1700. The refinements are in the details: the cast-iron frame that holds 20 tonnes of string tension, the precision of the repetition lever, the careful regulation of each escapement, the voicing of the felt. The mechanism is old; the precision is modern.

Flow diagram labeled How a piano note sounds: six labeled boxes connected by arrows reading Key pressed → Wippen rises, jack launches hammer → Escapement releases, hammer in free flight → Hammer strikes string, rebounds → String vibrates; bridge transfers energy to soundboard → Soundboard radiates sound; below, two branches labeled Damper lifts while key is held and falls on release, and Pedals modify which dampers are raised

The complete chain from key press to sound. The escapement releases the hammer before string contact; the string transfers energy through the bridge to the soundboard; the damper controls when the note stops. The pedals act on the damper system independently of the keys.

Sources

Read this article online: https://tissimo.co/articles/how-a-piano-works-hammers-strings-soundboard-and-pedals