My theoretical purpose is to distinguish between musical allusions and musical co-presences. A musical allusion is usually a citation of a melody or phrase from a work by a predecessor. In Shostakovich's Fifteenth Symphony, for example, references to Rossini's William Tell and to Wagner's Ring have tantalized listeners and provoked speculation on what these allusions could mean. In general, Shostakovich's music is notable for containing numerous allusions, often ironic, to previous composers. By contrast, Shostakovich's 24 Preludes and Fugues and the Eighth String Quartet show more than mere allusions to Bach: the very methods and forms of the Baroque master are incorporated into the later composer's works, as if Bach were a co-creator with Shostakovich; and those methods and forms are not employed ironically. That is to say, the music of Bach has a special significance for Shostakovich, a presence that goes beyond any topical value.