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The time signature is 2/8, and the piece is played with triplets but then at bar 17 the top triplet is replaced with a quintuplet with stretto marked above. I am a bit confused how to play this.
What type of polyrhythm is this? and what does the stretto marking mean in this situation?
Thanks
TL;DR: This is a 6:5 (left-hand against right-hand) polyrhythm, intended to create rhythmic and metric tension. The stretto reinforces this.
For illustration, here is m. 18:
(IMAGE SOURCE: IMSLP, Mikuli Edition)
The "5" refers to the entire right-hand part, which should be played as five sixteenth notes across the two-beat bar. The notation shown in the video edition is incorrect, which is what makes it confusing.
The effect of the five-tuplet is to create a "slow-motion" feel compared to the double-triplet feel of the preceding measures. There is a sense of rhythmic or metric stretching or delay, and this is strengthened by the stretto marking. Chopin is saying to make these measures a particularly tense or emotionally fraught portion of the Prelude.
NOTE: The left-hand part should remain a triplet. This also assists in creating tension by "arguing" with the right-hand part, trying to pull it forward rhythmically.
The way to know it remains a triplet is the note grouping: 3 sixteenths followed by an eighth rest (two sixteenths). If a five-tuplet were intended, the grouping would follow the right hand's two sixteenths, then three sixteenths (two sixteenths, one sixteenth, eighth rest [or two sixteenth rests, perhaps, for clarity]).
