The Japanese prosody term for the sound units counted in haiku and tanka is *on*.
In traditional Japanese poetry, *on* are the rhythmic units used to measure a poem’s pattern. A standard haiku is conventionally described as 5–7–5 *on*, while a traditional tanka is commonly described as 5–7–5–7–7. English discussions often translate *on* as “morae,” although older poetry writing may use terms such as *onji*.
The units are not identical to English syllables. A long vowel, a doubled consonant, or the syllabic nasal can contribute separate counts. For example, Japanese words that look short when written in romanization may occupy more *on* when represented in hiragana.
This distinction explains why directly copying a 5–7–5 syllable pattern into English does not reproduce Japanese meter exactly. Modern Japanese linguistic terminology also uses *haku* or *mōra*, but *on* remains the traditional term encountered in explanations of Japanese poetic form. The distractors are related poetry concepts: *kigo* means a seasonal word, *kireji* a cutting word, and *renga* linked verse.