About Tanka and Its History by Amelia Fielden

"The waka or tanka is an unrhymed verse form of thirty-one syllables or sound units¹ most often written in one continuous unpunctuated line. Nearly all Japanese syllables consist of a single vowel, or consonant plus vowel. As the language has only five vowels, rhyming is too simple to be interesting, hence Japanese poetry does not depend on rhyme. There are no poetic stress accents, so metre based on stress is not possible, either. Instead, traditional Japanese poetry is given rhythm by writing to a pattern of 5/7/5/7/7 sub-units or sound sets, with varying breath pauses being made when read aloud. Japanese is an agglutinative language which strings together shorter elements to create long, sometimes complex, word and phrase formations. Rhythmically and semantically, 5/7/5 combines unevenness with alternation, thus providing a natural balance to offset its inherent fluidity."

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