What is the difference between overdamped and underdamped pianos?

The damping of a piano is the mechanism that stops notes ringing on once you have taken you finger off the key. This function can be overridden by depressing the sustain pedal which stops the felt dampers returning to the string, allowing the note to ring on once the finger is lifted.

What is the difference between a piano that is overdamped and one that is underdamped?

Today all new pianos are built with underdamping as standard. The difference mechanically is that with overdamping there is a damping rail that sits above the hammers. The dampers are positioned near the top end of the string so the damping effect is less effective than underdamped instruments. If you lift the lid of an upright piano and you see a wooden bar with the hammers sitting below, it is overdamped.

The damper felts on an underdamped piano are beneath the hammers and therefore further towards the centre of the string so deaden the string instantly as the finger is removed from the key. The result is much clearer definition of notes with less ringing on of previously played notes. When you lift the lid of an underdamped piano you will have an unobstructed view of the hammers with felt dampers sitting below.