Cristofori's action already embodied many of the principles still found in 2000-era actions. With the piano, a hammer strikes the string, whereas with a harpsichord, a mechanism plucks the string. Other than in the action, the first pianos were quite similar in construction to contemporary harpsichords.

The piano action was the important innovation that Bartolomeo Cristofori created when he invented the piano in 1698. The action, in short, is what makes a piano playable or not to an individual musician.' Roughly speaking, a piano's action is light when its keys fall easily under the fingers, and heavy when a noticeable downward thrust is required. 'A professional pianist is likely to care most about the piano's action, because that is what controls its responsiveness and relative lightness-or heaviness-of touch. The design of the key action mechanism determines the 'weighted keys' feeling that is, the feeling of the heaviness of the touch of the keys. Action can refer to that of a piano or other musical keyboards, including the electronic or digital stage piano and synthesizer, on which some models have 'weighted keys', which simulate the touch and feel of an acoustic piano.

The piano action mechanism (also known as the key action mechanism or simply the action) of a piano or other musical keyboard is the mechanical assembly which translates the depression of the keys into rapid motion of a hammer, which creates sound by striking the strings.

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