Keyboard instruments come in many sizes and shapes but all use a keyboard as a control interface: a row of levers pressed by the fingers. The piano is the most common keyboard instrument.
Keyboard | Control interface for piano, organ and related instruments
Sounds from the keyboard family vary greatly: pipes, reeds, plucked strings, hammered strings, bells and electronics. When classified according to sound production, pianos are chordophones due to use of vibrating strings, whereas pipe organs are aerophones because of the vibrating air columns within pipes.
Most of the keyboard family is capable of producing both melody and harmony simultaneously, making them a favorite tool of arrangers and composers.
Harpsichord | Alessandro Trasuntino, c. 1531 | Harpsichords were a prestige object and popular in households during the Renaissance and Baroque. | Royal College of Music Collection
Harpsichord
The harpsichord precedes the piano by centuries but shares a similar form factor. A plectrum mechanism (quill) plucks harpsichord strings, whereas pianos strike the strings with a hammer. Harpsichord tone is softer, shorter in sustain and more metallic than the piano. Harpsichords are incapable of graded dynamics (e.g., crescendo and diminuendo) since the keys do not respond to pressure.
Introducing the Harpsichord | Mr. Devine discusses the harpsichord (2:55)
Piano
The piano was invented during the early eighteenth century by Italian luthier Bartolomeo Cristofori (1655-1731). Although similar in appearance to a harpsichord, the piano uses felt hammers to strike the strings instead of plucking with a quill. Hammers produce a darker tone and longer sustain than quills.
The original name was piano e forte (soft and loud), because, unlike the harpsichord, it was able to produce dynamics from piano to forte. Dynamics are controlled by striking the key harder or softer. Eventually the name was shortened to pianoforte but is called the piano (soft) in North America albeit modern designs are louder than ever!
Étude in C Minor, Op. 10 No.12 (Revolutionary) | Fréderic Chopin (2:34)
Organ
The oldest keyboard instrument, the organ, uses air flowing through pipes to create sound, qualifying it an aerophone. The first pipe organ was the water powered hydraulis, invented in Greece during the third century BC. Nero (37-68 CE), a Roman emperor and persecutor of Christians, introduced the organ to Rome in 67 CE. Organs were loud enough to be played in coliseums and quickly became a status symbol of the Roman upper class. The instrument eventually spread throughout Europe and the Middle East. By the fifteenth century, organs were a common installation in Christian churches.
Organ Pipes | Maria della Vittoria, Rome, Italy | Peter Kun Frary
Most pipe organs have multiple keyboards for the hands and a pedal keyboard for the feet. Multiple keyboards allow playing of multiple banks of pipes. Each bank of pipes has a different timbre such as flute, reed, brass, etc. Banks of pipes may be switched by opening and closing knobs called stops. Organists mix pipe timbres for expression and dynamic control.
Organ pipes need air to sound and, prior to electric pumps, air flow was created for large church organs with bellows pumped by little boys inside the organs!
Organ (1692) | A German household organ (regal) built by Simon Bauer. To produce sound, somebody pumped the bellows. | Museum of Fine Arts
Electronic musical instruments—electrophones—produce sound with electronics: an electrical signal is output, processed and amplified through an audio system.
Electric Guitar | Las Vegas | Peter Kun Frary
Modified Acoustic Instruments
Most electronic instruments are modified acoustic instruments, e.g., electric guitar, electric violin, electric 'ukulele, etc. A mechanical vibration such as a plucked or bowed string is processed and amplified via electronics.
Dimmar Öldur Rísa | Gulli Bjornsson | Performed by JIJI | The electric guitar is one of the most common electrophones. (9:12).
Synclavier PSMT (1984) | The Synclavier was the first really good digital music production work station and was used extensively in popular, jazz and movie scores during the 1980s and 1990s. | Wikimedia Commons
Purely Electronic
Some electrophones are purely electronic, as is the case of computers, samplers and synthesizers. That is, there are no mechanical vibrations to mic and process. Musical sounds are generated solely within the circuits of the electrophone.
Purely electronic musical instruments were not common until the second half of the twentieth century and had little use in classical and traditional music. However, they are important in popular music, film scoring and computer games.
Invented in 1920 by Leon Theremin, the theremin (originally, thereminvox) is the first purely electronic instrument to gain popularity. The performer moves his or her hands and fingers in the air to control pitch, duration and dynamics. The relative position of the performer's hands are sensed by two antennas (proximity sensors) which, in turn, control oscillators for pitch and loudness. Electrical signals from the oscillators are amplified and sent to an audio speaker.
In the Les Berceaux video below, Carolina Eyck uses her right hand to change pitch, whereas her left hand controls volume.
Les Berceaux (Gabriel Fauré) | Carolina Eyck, theremin, and Christopher Tarnow, piano (3:03).