A microtone is any interval that is noticeably smaller than a semitone (half tone). Microtones are found in Indian, Arabic, and Eastern European
music, and certain exotic scales. They have also been exploited by some
later nineteenth- and twentieth-century composers.
Quarter-tone pianos have been constructed, but the readiest means of producing microtonal music is using bowed instruments (on which, as in vocal music, are even sometimes produced accidentally). Blue notes are indeterminate microtonal notes used in blues music.