To me it seems like microtones are covered in MusicXML, e.g. <pitch> <step>E</step> <alter>0.5</alter> <octave>5</octave> </pitch> (From the Lilypond test suite, example "01d-Pitches-Microtones.xml") The MusicXML DTD, "note.mod" says: "Decimal values like 0.5 (quarter tone s...