The traditional number of syllables in a haiku is 17, arranged in a 5–7–5 pattern. More precisely, classical Japanese haiku count 17 morae, or sound units, rather than syllables in the strict English-language sense.
Haiku developed from the opening stanza of renga, a collaborative Japanese linked-verse tradition. That opening stanza was called a hokku and followed the short–long–short rhythm that later became associated with haiku. The modern term “haiku” was introduced by the Japanese writer Masaoka Shiki in the late 19th century.
The familiar 5–7–5 rule is a useful traditional guide, but it does not apply identically in every language. Japanese morae and English syllables are not equivalent, so a direct translation may not contain exactly 17 English syllables while still reflecting the original form. Traditional haiku also commonly include a kigo, or seasonal reference, and a kireji, or cutting word. Contemporary haiku writers often use freer structures.