An enharmonic keyboard is a musical keyboard based on an enharmonic scale. At the very least such keyboards will have 17 keys per octave, and enharmonically equivalent notes will have different pitches. A typical keyboard will have one key for, for instance, C sharp and D flat, but a basic 17 key enharmonic keyboard will have two different keys for these notes. Traditionally, these keyboards use split sharps to express both notes.
One of the first instruments with an enharmonic keyboard was the archicembalo built by Nicola Vicentino, an Italian Renaissance composer and music theorist. The archicembalo had 36 keys per octave and was very well suited for meantone temperament. Vincentino also built an organ equipped with an enharmonic keyboard, the archiorgano.
Many instruments with enharmonic keyboards were built during the Renaissance and Baroque eras. Most composers and performers who used these instruments are virtually unknown today. Among them are Johann Kaspar Kerll's teacher, Giovanni Valentini, who played a harpsichord with 77 keys for 4 octaves (19 keys per octave plus one extra C), and Friedrich Suppig, who in 1722 published one of the definitive works for an instrument with an enharmonic keyboard: the Fantasia of the Labyrinthus Musicus, which is a multi-sectional composition that makes use of all 24 keys and is intended for a keyboard with 31 notes per octave and pure major thirds.
With the advent of microtonal music in the 20th century, instruments with enharmonic keyboards became more fashionable, as did early and Baroque music for such instruments. For performance and recording purposes, either old instruments are reconstructed or two recordings of two differently tuned instruments are combined in one, thus creating an effect of an enharmonic keyboard.
Isomorphic note-layouts are a class of enharmonic keyboard. One isomorphic note-layout, the Wicki, when mapped to a hexagonal array of buttons, is particularly well-suited to the control of enharmonic scales. The orientation of its hexagonal columns of octaves and tempered perfect fifths place all the notes of every well-formed scale -- pentatonic (cardinality 5), diatonic (cardinality 7), chromatic (cardinality 12), and enharmonic (cardinality 19) -- in a tight, contiguous cluster. The notes of each progressively-higher cardinality are appended to the outer edges of the lower-cardinality scale, such that each well-formed scale's note-controlling buttons are embedded, unchanged, within the set of those controlling the higher-cardinality scales. Hence, the skills gained in learning to play chromatic music on a chromatic Wicki keyboard can be applied, without modification, to performance on an enharmonic Wicki keyboard. Isomorphic keyboards were not discovered until the latter half of the 19th century. Had they been discovered earlier, music history could have been quite different.