Haiku is a form of Japanese poetry, consisting of 17 Japanese on (a phonetic unit identical to the mora), in three metrical phrases with the number of syllables of 5 (first line), 7 (second line), and 5 (third or last line) or on respectively, and typically containing a kigo, or seasonal reference. In Japanese, haiku are traditionally printed in a single vertical line, while haiku in English usually appear in three lines, to equate to the Japanese haiku’s three metrical phrases. Previously called hokku, it was given its current name by the Japanese writer Masaoka Shiki at the end of the 19th century.

In Japanese haiku a kireji, or cutting word, typically appears at the end of one of the verse’s three metrical phrases. While difficult to precisely define its function, a kireji lends the verse structural support,[3] effectively allowing it to stand as an independent poem. Depending on which cutting word is chosen, and its position within the verse, it may briefly cut the stream of thought, suggesting a parallel between the preceding and following phrases, or it may provide a dignified ending, concluding the verse with a heightened sense of closure.

In English, since kireji has no direct equivalent, poets sometimes use punctuation such as a dash or ellipse, or an implied break, to divide a haiku into two grammatical and imagistic parts. The purpose is to create a juxtaposition, prompting the reader to reflect on the relationship between the two parts.

A haiku traditionally contains a kigo, a defined word or phrase which symbolizes or implies the season referenced in the poem.

Among traditionalist Japanese haiku writers, kireji and kigo are considered requirements; yet, as noted above, kireji are not used in English. Kigo are not always included by modern writers of Japanese “free-form” haiku and some non-Japanese haiku.

In contrast to English verse which is typically characterized by meter, Japanese verse counts sound units (morae), known as “on“. The word on is often translated as “syllable”, but there are subtle differences between an “on” and an English-language “syllable”. Traditional haiku consist of 17 on, in three metrical phrases of 5, 7, and 5 on respectively.

The word onji (音字; “sound symbol”) is sometimes used in referring to Japanese sound units in English[5] although this word is archaic and no longer current in Japanese.[6] In Japanese, the on corresponds very closely to the kana character count (closely enough that moji (or “character symbol”) is also sometimes used[6] as the count unit).

One on is counted for a short syllable, an additional one for an elongated vowel, diphthong, or doubled consonant, and one for an “n” at the end of a syllable. Thus, the word “haibun”, though two syllables in English, is counted as four on in Japanese (ha-i-bu-n).

Most writers of literary haiku in English use about ten to fourteen syllables, with no formal pattern.