When two or more subject entries overlap in time.
During the formal exposition of the subject at the beginning of a fugue, each voice takes a turn presenting the subject. For clarity when establishing familiarity through repetition, no voice starts on a new subject entry until the previous voice has finished. This is essentially the only important formal rule in the historical construction of fugues. It makes sense logically and artistically. Here is a diagram of a standard exposition.
This very expectation of an orderly series of uninterrupted subject entries creates the drama of increased tension when in fact it doesn't happen. Stretto refers to a section of a fugue where subject entries overlap, the second beginning before the first has completed. The first fugue from Bach's Well-Tempered Clavier, Book 1, No. 1 in C major is noteworthy for its extreme use of stretto:
There are two chief effects of stretto: the action and the movement of time accelerates, and the texture appears to get more dense. Neither is necessarily true. Each subject entry remains the same in length (is completes) and, depending on the fugue, the all voices may already be active. But compared to a formal exposition, the time to complete full exposure across voices is compressed. And because multiple parts are voicing the primary material of the fugue (the subject) simultaneously, the dramatic density has increased: more voices are speaking of fundamentally important matters.
The general effect is excitement, acceleration, fuller realization, a certain indescribable ecstasy with the sensation of heightened simultaneity. Stretto is used quite frequently throughout the fugues of Bach and Shostakovich. It appears often near the end for a dramatic finish but may just as easily be found in the middle or not long after the exposition. The duration of stretto varies from short, maybe a few subject entries, to long, to the extent that some fugues are saturated with stretto, becoming adventures in different degrees of stretto alone, often in the absence of other technical devices. These are often called stretto fugues, the most noteworthy being:
Bach - WTC - Book 1 - No. 1 - C major
Bach - WTC - Book 2 - No. 5 - D major
Bach - WTC - Book 1 - No. 22 - B-flat minor
Shostakovich - Op. 87 - No. 22 - G minor
Remember that subject entries need not be complete: many fugues feature false entries where the incomplete subject breaks off into counterpoint. Frequently, this occurs in conjunction with, as if because of, a new voice entering with the subject, itself perhaps only fragmentary. This creates the effect of stretto, and technically, it is. But in this case, the subject entries are incomplete: this would be a lesser form of stretto during an episode.
Stretto can be used in conjunction with other transformations such as inversion. Stretto may be applied to melodic lines other than the subject: countersubjects, motives, etc.
When two more subject entries run at exactly the same time.
A natural extension of overlapping subject entries is the possibility of completely simultaneous or parallel subject entries: multiple voices state the subject together in a unison texture. At first, this seems like the tightest stretto possible. But parallel entries are quite special. First, without the tension of staggered entries or interruption, parallel entries have a particular repose. The contrapuntal texture is diminished in density since multiple voices work in complete unison. The is, in a sense, the complete opposite of counterpoint. As an effect, it is certainly the opposite of stretto. With parallel entries, the subject is amplified, strengthened, emphasized by the unification of multiple voices into a single chorus. Bruhn uses the wonderful term "Exalted". Parallel entries almost always signify the zenith of a fugue's development: it represents a climactic completion, a profound resolution. Though not rare, the parallel entry is infrequent since because of its reductive effect on counterpoint.
Though parallel entries have a unison rhythm, they need not be in terms of pitch. There are two possibilities. If the voices are simply the same note (e.g. C) but separated by octaves, they form a unison of pitch without harmony, the most blended and bold unity possible. The other possibility is for multiple voices to differ by intervals other than the octave so that together, they create harmonies. This has a different feel and a different expressive purpose.
Shostakovich uses parallel entries in novel and powerful ways. Many of his parallel entries exploit the amplified quality of voices in unison as a dynamic or "orchestral" element in his compositions. That is, the subject entry is indeed one thick, broad and bold entry, standing out for its color and its force. He also uses parallel entries for the sake of rich harmony, exploiting the blend of parallelism at non-octave intervals for the homophonic chords they create.
Here is a visual representation of a pronounced example of parallelism from Bach's Well-Tempered Clavier, Book 2, No. 16, in G-minor. Notice that there is even an occurrence of parallel countersubject entries.
Like stretto, parallelism can occur with countersubjects, second subjects and the like. Notice in the example above that the inverted subject is also treated to parallel entries.
Credit goes to Siglind Bruhn for her excellent diagrams.
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