Permeable boundaries form the musical ‘thread’ of Don Giovanni– a compositional strategy fundamental to the opera's character. Customary cadential borders get omitted or blurred; material heard early in the opera prominently returns; and all the accompanied recitative-set piece pairs act as ‘composite pieces’ – scenes in which musical material as well as dramatic function bind the accompanied recitative and aria or duet together and fuse them into one entity. ‘Permeability’ is heightened in Don Giovanni due to the supernatural elements of the plot, the title character's refusal to to submit to society's strictures, Gluck's association with the story, and Mozart's propensity for musical one-upmanship. Yet it is by no means unique to that work. Studying the relationships between accompanied recitatives and adjacent numbers reveals a ‘middleground’ of musical continuity that lies between long-range tonal plans and the motivic and tonal unities of individual numbers. Hence these passages challenge, as well as complement, some of our underlying assumptions about operatic form, and urge us to expand our definition of a ‘number’.