Structural level, Schicht. One of Schenker's most important concepts, that the events of a tonal composition are ordered in a hierarchic manner. The most important structural events occur at the background, and form the fundamental structure. Events of more local importance occur at higher structural levels, the middleground and foreground, and the ornamental and motivic detail occurs at the surface. Schenker contends that, in general, all structural levels follow basic principles of harmony and counterpoint. The progressions of the higher levels are means of prolonging, or composing out, the fewer but more important events of the deeper levels.
The example, from Variation 13, shows the middleground and foreground levels. The foreground is more detailed, but by no means includes every note of the surface of the music.