omnitonic
/ˌɒmnɪˈtɒnɪk/Music. Now historical
adjective
1Designating or relating to a musical instrument, especially a horn, which is so constructed as to allow it to be played in any key.
2Relating to all tones or tonalities; specifically relating to or designating a theory of harmony which allows for continuous modulation between keys.
Origin
Mid 19th century; earliest use found in The Times. From omni- + tonic, after French omnitonique.