Scale-step, Stufe. Scale-step refers to Schenker's idea of an abstract harmonic function, which may exist in a single chord or comprise several harmonies. Stufen represent the underlying harmonic progression of a composition. "The scale-step asserts its higher or more general character by comprising or summarizing the individual phenomena and embodying their intrinsic unity in one single triad." (Schenker, Harmony, p. 139.) In the text, Stufe is translated in a variety of ways, depending on context.
In the example below, from the Aria, there are two scale-steps,
I and V. I, which extends to the middle of m. 4, includes several subordinate harmonies (shown in parentheses) that develop out of the elaborations of the melody, as Schenker illustrates in his discussion of the Aria.