undamental Structure, Ursatz. The background tonal structure of a composition, comprising an Urlinie (fundamental line) and a bass arpeggiation (Bass-brechung). There are three forms of the fundamental structure in Schenker's theory, based on descending Urlinie from the third, fifth, and octave to the tonic. The fundamental structure of Brahms's Op. 24 Variations is shown at the bottom of this page.