Imitative Entries, Canon, and Fugue

A single melody, the subject, states itself alone in one voice. Before it finishes, a second voice enters with the same melody, usually transposed up a fifth or down a fourth, called the answer. The first voice does not stop and wait. It keeps going with new material, called the countersubject or free counterpoint, woven against the answer. This is imitative counterpoint: the voices are independent lines, but they share the same thematic idea entering at staggered times, so the texture thickens one voice at a time instead of all at once.

A fugal exposition is the opening section where every voice gets its turn. In a four-voice fugue, voice one states the subject in the tonic, voice two answers a fifth higher (or a fourth lower) while voice one continues, voice three re-enters with the subject back in the tonic, and voice four answers again at the fifth while the others keep spinning independent lines. Each entrance is spaced one subject-length after the previous one, so a four-measure subject means the fourth voice enters twelve measures after the first. Find each entrance by counting subject-lengths forward from voice one.

This is polyphonic texture from the fundamentals unit taken to its most organized extreme. Where a general polyphonic texture just means multiple independent melodic lines at once, fugue adds the rule that those lines are built from one shared idea passed around in turn. That shared identity is what makes staggered entrances sound like conversation rather than noise: each voice restates something the listener already recognizes, while the surrounding lines keep moving independently.