The piano, that common instrument of school music programs, appears to be the ultimate expression of the stringed musical instrument, which date back to the lyre and the harp. Pianos (a shortening of the compound term "piano-forte") work by striking wires with felted (or leather headed) hammers, with a redaction mechanism that pulls the hammerhead away from the wire before it can dampen out the sound. Because the force of the hammer strike is generally proportional to the stroke on the key, this allows a piano to play a note softly (piano) or loudly (forte), leading to its name.
The immediate predecessors of the pianoforte were the clavichord and the harpsichord, both of which tried to combine the ease of play of a traditional organ keyboard with the expressive range (and general portability) of a large concert harp. Of the two predecessor instruments, the harpsichord was the more common, and used a mechanism that plucked the strings (and later wires) of the instrument when a key was hit. This lead to a distinctive "plinking" or "plucking" sound, more like a strung harp, but lacked the ability to adjust the volume of a given note, and had only limited ability to change the duration of a note.
While the harpsichord provided the mechanism for tying keys to striking particular strings that was used to make the first pianos, the clavichord (an earlier instrument, invented in the 15th century, was the first keyboard instrument to strike the strings by key stroke, hitting them from the side with a small (dull) blade called a tangent. Clavichords fell out of favor in the 17th century, and were virtually unheard of from roughly 1750 to 1890, when a number of musical instrument shops began making them again as a smaller complement to the piano.
Prior to the clavichord, the first real stringed instrument that used hammers was the dulcimer, with variations such as the cymbalon and the readis spreading through the Balkan regions. All of these instruments relied on the player to strike strings with small hammers, often times holding multiple hammers with different heads in the gaps between their fingers, to get different tonal ranges, including a felted head for dampening a string.
The first true pianoforte was built in 1700 by Bartolomeo Cristofori of Padua, Italy. His patrons, the Medicis, commissioned the first ones; there are three Cristofori pianofortes still in existence, dating back to the 1720s.
Cristofori's original pianos had several differences from the modern pianos we've come to expect - for one, they only covered four octaves, rather than the modern piano's seven-and-a-half octaves. Second, because of the materials used at the time, it was considerably softer in sound than the modern instrument. Third, it had no damper pedals for lifting the dampers from the string. The damper pedals were an invention of Gottfried Silbermann, who made near direct copies of the Cristofori piano otherwise, and tried to get Johann Sebastian Bach interested in the instrument for compositions and concert performances.
Bach was notably unimpressed with Silbermann's early pianos, claiming that the upper range would be too quiet to make an effective concert hall instrument. While this brooked a fair bit of animosity between the instrument maker and the reknowned composer and concert artist, in the end, Bach was right. It wasn't until 40 years later that Bach actually endorsed the creation of a piano, mostly after Silbermann's apprentices worked on variations of the design.
Nearly from its inception, the main driving force in the evolution of the piano was to make it louder and brighter in the high notes. Several innovations have been incorporated into the design to do this. Among them include more precise mechanisms for swinging the hammers, high tensile steel replacing the catgut strings, and changes to the surfaces of the hammers and their materials, plus innovations in resonator and fretboard spaces to give the instruments a greater range, such as the double key escarpment, which allowed a note to be repeated even if the hammer hadn't risen back to its full resting position.
The high point of piano evolution happened in the 19th century, with the development of felted hammers (allowing higher string tension), better quality steel for the wire, iron frames on the sounding board, and several other innovations of note; it was in the late 19th century that the upright piano was perfected, allowing the piano to move from the concert hall to the parlors of the middle class, where the ability to play the piano was a sign of culture and refinement, a place that it still holds today, albeit to a much reduced extent.
Kevin Sinclair is the publisher and editor of MusicianHome.com, a site that provides information and articles for musicians at all stages of their development.