Broad System of Ordering A general subject classification scheme, commissioned by UNESCO, intended to be a "switching language" among existing classification schemes and thesauri, to make them mutually compatible on a general level. It provides about 4,000 not very detailed subdivisions. (ALA Encyclopedia) It was chosen for the Digital Library because of its attempted universality, because it was already in electronic form, and because it was not expensive. Used by the project ot provide basic subject indexing.