enharmonical

 Of or pertaining to that one of the three kinds of musical scale (diatonic, chromatic, enharmonic) recognized by the ancient Greeks, which consisted of quarter tones and major thirds, and was regarded as the most accurate.  [1913 Webster]

 Pertaining to a change of notes to the eye, while, as the same keys are used, the instrument can mark no difference to the ear, as the substitution of A1913 Webster]