Many (almost 50) years ago, studying classical guitar from a Mr. Belson (originally Bellisoni; he was Italian) in St. Paul, Minnesota, for singing-accompaniment purposes, he had me singing in fixed-Do format, Do Re Mi Fa Sol La Si Do. It seems that "Ti" is for the movable Do system that many of us learned in grade school, but "Si" is for fixed Do.

Dave Oesterreich