01. Form / The 5-7-5 rule
Three lines of five, seven, and five — a convention borrowed, imperfectly, from the Japanese on.
The on is a sound unit roughly equivalent to a syllable but not identical — which is why some poets and translators argue against strict 17-syllable English haiku. Either way, 5-7-5 is what most readers and editors expect, and that's what this instrument checks.