The second subject entry in a pair of entries during the exposition.
The exposition of a fugue is essentially the only part of a fugue governed by standard rules. The simplest description of the exposition is that each voice in the fugue enters, one at a time, with a complete subject statement. Give a fugue for four voices, the exposition will be a series of at least four subject entries, one and only one per voice. This would appear to be a strict and rather direct form of imitation.
If you look or listen closely to an exposition, you will discover that the second subject entry is in fact not a literal imitation of the first subject entry. The most important difference is that the second entry must begin on a different degree of the scale than the first entry. Most often, the subject used in the first entry is transposed up a perfect fifth (or, to the same note an octave lower, down a perfect forth). The two entries form a pair. The first entry is called the "leader" ( in Latin, dux), and the second the "follower" (comes). The standard English term for the follower, the second subject entry, is the answer.
Here is a vivid example from Shostakovich's Op. 87 Fugue No. 7 in A major. In this fugue, the subject uses only tones from its tonic triad, A major. The answer is the same melody transposed to the 5th degree of the A major scale, E (in this case, down from A, down a perfect 4th). The answer uses only tones from the E major triad.
Subject (partial) - starts on A, the tonic
Answer (partial) - starts on E, the dominant
This is often called imitation at the 5th, a technique used in most fugues. The key point is to create a harmonic contrast and diversity, giving the fugue rich tonal momentum. Many inventions lack this complexity with imitation occurring on the same note but in a different octave.
Here is another simple example of a complete subject and answer, this time from Bach's Well-Tempered Clavier, Book 1, No. 17, A flat major.
Subject - starts on A, the tonic
Answer - starts on E, the dominant
If you look or listen closely, you will detect additional modifications: the answer is not the literal transposition of the subject down a fourth. The second note of the answer has been slightly modified in pitch. Depending on the details of the subject and the personal taste of the composer, the answer is often adjusted from a literal transposition to something that satisfies the tonal properties of the fugue: it sounds better. This is a complex technical issue and one of considerable art. It can involve modifications of the subject and even imitation at a different interval than the 5th. Suffice it to say that the answer is either real, meaning a literal transposition, or tonal, where adjustments are made.
In this example, Shostakovich's answer is real; Bach's is tonal.
Regardless of the number of voices or the technical details of the answer, the concept of and necessity for the answer creates a pairing of subject entries, a call and response, a two-part musical narrative that characterizes the exposition. In many fugues, this phenomenon goes beyond the exposition so that many subject entries throughout the fugue exhibit a pairing, a powerful expectation and source of satisfaction for the listener.
There are a few additional implications of the answer worth highlighting. First, the idea of imitation at a different pitch (usually a 5th) extends to the second entry of every pair during the exposition. While the second entry is a 5th away from the first entry, the third entry should be a 5th away from the second entry. Typically, this means that the third entry starts on the same pitch (disregarding octaves) as the first entry: back to the beginning. The second, natural implication of the transposed answer is that countersubjects during the exposition must also be transposed from their first appearance to their second, etc.
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