High Baroque Fugal Exposition

A fugue is a contrapuntal composition in which a short melodic idea — the subject — is introduced and then imitated successively in each voice. The opening section, called the exposition, presents the subject in every voice exactly once. The first voice states the subject alone in the tonic key. The second voice answers in the dominant, while the first voice continues with a new melody called the countersubject. The third voice restates the subject in the tonic, and the fourth (if present) answers again in the dominant, with all previously entered voices providing counterpoint above or below.

The answer in the dominant may be either real or tonal. A real answer transposes the subject exactly up a fifth (or down a fourth). A tonal answer makes small adjustments — usually changing the opening leap of a fifth to a fourth — to keep the harmony firmly rooted in the tonic-dominant axis rather than modulating to the dominant's dominant. Tonal answers are required when the subject begins on scale degree 5 or emphasizes the dominant pitch prominently, since a real answer would shift the tonal center too far.

The countersubject is a melody designed to sound well against the subject in double counterpoint — meaning it can appear above or below the subject interchangeably. This invertibility is what allows the countersubject to serve as a stable accompaniment throughout the fugue as the subject migrates through different voices and registers. Short transitional passages called episodes, built from sequences and motives derived from the subject, connect later entries of the subject in different keys. The exposition is thus the fugue's statement of its motivic and contrapuntal DNA.