How a piano action works: key, hammer and damper explained

01From keystroke to jack

When you press the front of the key down, its rear end rises. That is where the jack sits, a small lever that passes the motion on to the hammer. The jack pushes against the hammer roller at the base of the hammer shank and accelerates the hammer toward the string. This handover point is exactly where it is decided how sensitively a piano responds to a soft or hard touch, which is why adjusting the jack is one of the most important regulation tasks on a piano action.

Just before the hammer reaches the string, the jack automatically slips out from under the hammer roller. This is not an accident but deliberate design: the hammer travels the last few millimetres freely and strikes the string instead of being pushed rigidly against it. Only this way does a clear, free tone result instead of a muted knock.

02The hammer strikes the string, and the back check catches it

The hammer head, covered in dense felt, strikes the string and immediately rebounds. Without a countermeasure it would bounce uncontrollably and strike the string several times, an effect piano technicians call blocking. To prevent this, the rebounding hammer falls into the back check, a padded rest that holds it still before it can strike again.

Only when the key is released does the back check let go of the hammer, and the jack returns to its starting position, ready for the next stroke. On grand actions and on higher-quality upright actions, a repetition spring additionally allows the hammer to become ready to fire again while the key is only partially released, enabling fast note repetitions such as those needed in virtuosic passages.

03The damper: why the note stops when you let go

At rest, the damper, also felt-covered, sits on the string and keeps it silent. During a keystroke, the damper lifts off the string just before the hammer actually strikes, so the string can vibrate freely. When you release the key, the damper drops back onto the string and ends the note.

The sustain pedal (right pedal) lifts all dampers at once, so notes keep ringing after the keys are released and strings can excite each other through resonance. On modern pianos, the dampers sit below the hammer line (underdamper action); on older instruments you sometimes still find an overdamper design with dampers above the hammers. Today the underdamper action is practically the universal standard because it works more precisely and is easier to maintain.

04Why the action determines the feel

How finely the jack, hammer roller, back check and damper are regulated against each other largely determines how a piano feels: light or heavy, precise or sluggish. This is exactly why pianos in different price ranges often differ more in action quality than in tone alone. A Yamaha B10 Klavier schwarz poliert offers a solid, well-regulated base action for entering acoustic playing, while a Kawai K-200 Klavier schwarz poliert stands out with finer adjustability and higher-quality materials at the contact points.

Anyone who practises regularly notices the difference most during fast repetitions and very soft touches, both of which place high demands on how the parts work together. Our Klaviere - Neu category gives an overview of available models with different levels of action refinement.

A single keystroke sets a small chain in motion: jack, hammer, back check and damper, each with a clearly defined job so the note starts cleanly and stops just as cleanly. This precision is also why pianos in different price ranges often differ more in feel than in tone.

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