A keyboard that uses a different arrangement of keys than the conventional piano/organ style keyboard employed by most synths. Although the concept of alternate keyboards isn't unique to synths, they are generally impractical to implement on instruments that are not electronic, and much of the work done on alternate keyboards has occurred since about 1995. Generally, the goal of an alternate keyboard is to (1) create a key layout such that the fingering for a given chord remains the same when it is transposed to a different key, or (2) provide a practical means of playing alternate scales, including scales that include microtones.
Keyboards which allow transposing a chord to any key without altering the fingering are called "isomorphic". The first such was the Jankó keyboard, developed in the 19th century. This uses alternate rows of keys, containing naturals and accidentals in an alternating pattern, which allows transposing without altering fingering by moving up or down a row. No Jankó keyboard is currently on the market, but C-Thru Music offers a variation called the Axis keyboard, which has an array of hexagonal keys. A chord can be transposed by moving it up and down or across the array, and the fingering remains the same.
A keyboard which allows playing of microtone intervals is the Tonal Plexus series made by H-Pi Instruments. It contains arrays of larger and smaller pushbutton keys for various intervals, and can load tuning tables for playing various microtonal scales. To communicate microtonal intervals via MIDI, it sends combinations of note on and pitch wheel messages for each note played. An older concept used by the Buchla 112 touchpad array, and the EML 300, is to have an array of keys that each trigger a control voltage source that is infinitely variable. This allows the keys to be tuned to any interval; in fact, the notes do not have to be in ascending order across the keyboard.