A few years ago, in 2005, the great musicologist Joseph Kerman published a book on Bach fugues called The Art of Fugue: Bach Fugues for Keyboard 1715 - 1750. Included is a CD with not only performances of the fugues discussed, but also scores for each. The first one he talks about is the first fugue in the Well-Tempered Clavier, Bk 1 which has preludes and fugues in every key. The fugue in C major is a masterpiece of composition. As Kerman points out, this fugue is all about stretto. The fugue is only 27 measures long. The subject has 14 notes--which is also what Bach's name adds up to when converted to numbers: B (2) A (1), C (3) H (8) = 14. The subject takes up a measure and a half. But in this brief piece, the subject is heard twenty-five times! How is this possible and why isn't it boring? It's not! The answer is the beautiful contrapuntal technique known as stretto. This is the past participle of an Italian verb meaning "tight" or "narrow". Normally when a fugue subject is presented, it is allowed to run its course before another voice enters with the same subject. In stretto, the second and perhaps third voice enters part-way through, before the first is finished. The subject is layered on itself. The trick is to do this without causing any unpleasant dissonances. In this fugue, Bach virtually exhausts the possibilities, having the subject enter over itself at different intervals and with different rhythmic spacing.
Luckily, one enterprising person took this all apart for us. In the following clip from YouTube, Signor Polyphony has recorded midi versions of each voice separately, with the score, then all four voices together, with the complete score, played by Glenn Gould.
If you can make out the slightly fuzzy measure numbers, measures 14 to 16 are a particularly interesting stretto with alto, tenor (one beat later), bass (three beats later) and soprano (two beats later) all coming in close succession. Stretto is just one technique used in fugue. In the next one, in C minor, Bach doesn't use any stretto at all.