The clavichord is the simplest string keyboard instrument, developed in the 14^th century and widely manufactured until the 16^th century. It enjoyed widespread success thanks to its reduced shape factor and portability, as shown in the examples in Figure 1. The clavichord action is a simple key lever with a hammer on one end, as in Figure 2(a). This mechanism, described in Table 1, is characterized by greater expressivity than most other keyboard instruments, by enabling volume dynamics, vibrato and control over sound attack and duration. This action, however, leaves limited space for a soundboard, and the extremely low volume of this instrument, together with always-on passive string damping, relegated it mostly to private use, while the harpsichord enjoyed its fame as a concert instrument.

The harpsichord family includes several musical instruments, from the smaller spinet (Figure 3(a)) and virginal (Figure 3(b)) to the majestic double-manual harpsichords (Figure 3(c)), all sharing the same key action mechanism. The

harpsichord action is based on the single lever transmission that is illustrated in Figure 2(b). When depressed, the key lifts a jack that plucks one or more strings with its plectrum and also contains a damper that lets the string resonate, as explained in Table 2. However, the expressivity of the harpsichord is limited when compared to the clavichord: there is almost no volume dynamics, and vibrato cannot be performed. However, the on/off string damping and higher volume (due to multiple strings and larger soundboards) made the harpsichord more successful than the clavichord until pianos replaced them both. Furthermore, instrument makers tried to overcome the limited volume dynamics by adding an additional manual (i.e. an additional keyboard action on top of the main one, as shown in Figure 3(c)), with each key of the upper manual being activated by depressing it or the corresponding lower manual key: in this way, the higher manual plucks a single set of strings with a lower volume, while the lower manual achieves a higher volume by plucking two or more strings with each key.

The greatest Italian contribution to key action mechanisms is Bartolomeo Cristofori’s invention of the fortepiano. Born in Padua in 1655, Cristofori was hired by Ferdinando de Medici, Grand Prince of Tuscany, in 1688, in order to service the court’s instruments. Why Prince Ferdinando decided to select an outsider (Padua was part of the Republic of Venice) rather than one of the many harpsichord makers in Florence is still not clear, and a very limited reports on Bartolomeo Cristofori’s life are preserved, mostly limited to some notes in the Giornale de’ letteratid’Italia that Francesco Scipione, Marchese di Maffei (generally

known as Scipione Maffei), wrote in occasion of his visit to Florence in 1709 (Montanari, 1991). Some researchers (Pollens, 2017) explain this choice with Cristofori’s growing fame as instrument maker as well as his possible work on the fortepiano prior to moving to Florence. Prince Ferdinando was an accomplished musician, as well as an enthusiast of horology and other complex mechanical devices, and these factors might have led him to support Cristofori’s research.

In Florence, Bartolomeo Cristofori worked in his workshop in the Uffizi’s Galleria deiLavori (Pollens, 2017), where he built different kinds of keyboard instruments. An inventory of Prince Ferdinando’s musical instruments in 1700 lists seven keyboard instruments by Cristofori: two spinets, a spinettone, three harpsichords (one upright) and “un Arpicimbalo di Bartolomeo Cristofori, di nuovainventione, che fa il piano e il forte, a due registriprincipaliunisoni” (an Arpicimbalo newly invented by Bartolomeo Cristofori, which plays piano and forte, with two main unison stops), which was probably the first fortepiano that he made (Montanari, 1991). Cristofori was a prolific instrument maker, and he kept improving his design until his death in 1731.

Cristofori’s original design, often called fortepiano, introduces sweeping changes by introducing additional levers to improve motion transmission, by guiding the hammer only partially to the string and by implementing a damping mechanism as illustrated in Figure 4 and explained in Table 3. With a shape factor similar to the concert harpsichord’s one, the fortepiano is characterized by a large soundboard, as shown in the example in Figure 5, resulting in a good volume and a full sound rich in harmonics. The improved control over the damper and the momentum-driven hammer enable expressivity with both volume and tone dynamics.

After Cristofori’s death, the fame of German and Austrian makers quickly eclipsed the Italian inventor’s success. Gottfried Silbermann was aGerman instrument maker who improved Cristofori’s design by adding a hand-operated damper lever and introduced the new design to many influential composers of the age, including Johann Sebastian Bach (Pollens, 1995). In the 18^th century, Johann Andreas Stein developed the Viennese (or German) hammer action, with an alternative hammer arrangement that improved the player’s control of the hammers and a damper pedal (Pollens, 1995). Stein’s pianos were played by Mozart and Beethoven, among others.

The introduction of a pedal that lifts all dampers independently from the keyboard mechanism also lets all the strings resonate while keys are released. These mechanical changes enabled the shift from early classical piano music, with strong Baroque influences, to romantic music, with composers like Chopin and Liszt integrating the novelties of the fortepiano in their own pieces.

The grand piano, shown in the example in Figure 6, is the state-of-the-art instrument for piano concerts. As previously indicated in Figure 4(b), the intermediate lever of a fortepiano was attached to both damper and jack. On the contrary, the grand piano action, in Figure 7, has an independent damping mechanism where the damper is lifted only halfway through the hammer run when the key is depressed. This action, detailed in Table 4 and in the scheme in Figure 8, is also characterized by a repetition lever that allows to repeat a note even if the key is not fully released. This lever made a breakthrough in piano history by enabling fast note repetition for both soft (piano) or heavy (forte) notes.

Grand pianos have three different pedal mechanisms that are independent from the key action and alter the sound by either providing a lower volume, releasing the dampers from all the strings, or operating the sostenuto mechanism. Pedal mechanisms are held below the keybed of the piano by the pedal lyre, which has structural components that are strong enough to withstand the large

forces exerted by the pianist’s feet (Reblitz, 1993). The left or soft pedal can operate as 1) a shifter, meaning that a lever from the pedal mechanism fits onto a notch in the key frame and moves the whole key action of the piano slightly to the right (i.e. by one string) so that when a key is depressed, only two of its three strings are struck, hence resulting in less sound volume and different tone quality; or 2) a lifter, that brings up the hammer rail slightly closer to the strings, thereby reducing the hammer travel distance and resulting not only in a lower volume, but also in an altered key feel to the pianist that could hinder playability. The right or sustain pedal is an independent mechanism that lifts the damper rail from the key action mechanism, allowing free resonance of all the strings while the pedal is active. Contrary to the sustain pedal, the middle pedal enables the sostenuto mechanism, which essentially sustains only the notes that are depressed at the moment that the pedal is depressed, allowing for any other future notes to be unaffected, independently from the pedal position (Reblitz, 1993).

The action of an upright piano is designed to fit tight space requirements rather than performance. Therefore, the string layout is vertical rather than horizontal, as illustrated in Figure 9, in order to fit a smaller body frame. The repetition lever of grand pianos is also not included. This design offers the advantage of reduced encumbrance and lower cost. However, its shorter strings and smaller soundboard result in a lower sound quality than grand pianos and the lack of a repetition lever can limit professional pianists in execution. The functioning of an upright piano action is reported in Table 5, and a mechanical scheme is shown in Figure 10.

Most upright or vertical pianos have two pedals (i.e. soft pedal and sustain pedal), but higher cost and quality uprights may also have a sostenuto, a bass sustain or a practice pedal in the middle (Reblitz, 1993). The pedal mechanisms are similar to those of grand pianos, adapted to the vertical arrangement of the strings.

The first electro-mechanical keyboards were developed in the late 1920s, and they reached the peak of their success between the 1960s and the 1970s. The main difference between an electro-mechanical and an acoustic instrument can be identified in the soundboard: while acoustic instruments use a resonance chamber to amplify the sound and enrich it with harmonics, electro-mechanical instruments acquire the soundwave vibration through a pickup and amplify it electrically. Most electric pianos use fortepiano, grand or upright key action mechanisms, sometimes replacing strings with reeds or tines. For example, the Fender Rhodes piano, shown in Figure 11, uses the key action mechanism shown in Figure 12, which is based on the fortepiano kinematic scheme in Figure 4(b), whereas the clavinet is designed as based on the clavichord mechanism in Figure 2(a). Despite the different functioning and sound of electro-mechanical keyboards, the key action presents no significant change from previous ones.

Modern synthesizers (often shortened as synths), such as the iconic Minimoog in Figure 13, employ a drastically different approach than action mechanisms: instead of generating sounds by striking or plucking strings, reeds, or tines, they depend on electronics for analogue or digital soundwave generation and processing. Thus, synth keys generate numerical values such as ON/OFF signals or velocity signals, which are integers between 1 and 127 under standard MIDI protocol. For this reason, a single lever with an electronic sensor is enough, with a spring (sometimes with added weight) to return the key up after release, as shown in Figure 14. This mechanism, detailed in Table 6, is independent from gravity, enabling vertical keyboard layouts. Furthermore, synth action is extremely affordable, lightweight, and compact. Its mechatronic nature, however, can limit the expressivity of the instrument, even if additional tools are often added to compensate, e.g. pitch bend and modulation levers, aftertouch sensors, low frequency oscillators, ADSR parameters, filter cutoff and resonance knobs (Vail, 2014). For this reason, an additional mechatronic control known as aftertouch is sometimes added to premium synth actions to enable pressure sensitivity. The aftertouch is achieved by adding a pressure sensor beneath the keys to record and transmit MIDI signals related to pressure variations on a depressed key, which can be associated to effects such as vibrato or tonal changes.