Keyboard expression is the ability of a keyboard musical instrument to change tone or other qualities of the sound in response to velocity, pressure or other variations in how the performer depresses the keys of the musical keyboard. Expression types include: Velocity sensitivity—how fast the key is pressed Aftertouch, or pressure sensitivity — the amount of pressure on a key, once already held down Displacement sensitivity—distance that a key is pressed down Keyboard instruments offer a range of expression types. Acoustic pianos, such as upright and grand pianos, are velocity-sensitive—the faster the key strike, the harder the hammer hits the strings. Baroque-style clavichords and professional synthesizers are aftertouch-sensitive—applied force on the key after the initial strike produces effects such as vibrato or swells in volume. Tracker pipe organs and some electronic organs are displacement-sensitive—partly depressing a key produces a quieter tone. Velocity sensitivity The piano, being velocity-sensitive, responds to the speed of the key-press in how fast the hammers strike the strings, which in turn changes the tone and volume of the sound. Several piano predecessors, such as the harpsichord, were not velocity-sensitive like the piano. Some confuse pressure-sensitive with velocity-sensitive. To avoid this confusion, pressure sensitivity is often called aftertouch. The MIDI standard supports both velocity and aftertouch. In general, only high-end electronic keyboards implement true pressure sensitivity, while most professional-quality electronic keyboards support velocity sensitivity. Cheaper electronic keyboards, such as toy electronic keyboards and basic learning keyboards manufactured by Casio and Yamaha in the US$100 price range, do not have velocity sensitivity, but instead a manually-adjusted note volume. Pressure sensitivity or aftertouch The clavichord and some electronic keyboards also respond to the amount of force applied after initial impact—they are pressure-sensitive. This can be used by a skilled clavichord player to slightly correct the intonation of the notes when playing on a clavichord, and/or to play with a form of vibrato known as bebung. Unlike in a piano action, the tangent does not rebound from the string; rather, it stays in contact with the string as long as the key is held, acting as both the nut and as the initiator of sound. The volume of the note can be changed by striking harder or softer, and the pitch can also be affected by varying the force of the tangent against the string. When the key is released, the tangent loses contact with the string and the vibration