Keyboards with alternative sets of keys

There are some rare variations of keyboards with more or fewer than 12 keys per octave, mostly used in microtonal music, after the discoveries and theoretical developments of musician and inventor Julián Carrillo (1875–1965).
Some free-reed instrument keyboards such as accordions and Indian harmoniums include microtones. Electronic music pioneer Pauline Oliveros plays one of these. Egyptian belly-dance musicians like Hassam Ramzy use custom-tuned accordions in order to play traditional scales. The small Garmon accordion played in the Music of Azerbaijan sometimes has keys that can play microtones when a "shift" key is pressed.

Frequencies of the audible range on a twelve and eight equal tempered scale