The subject of the haiku usually relates to nature. Topics include natural elements such as grass, trees, seasons, animals and insects. Haiku is not limited to the subject of nature, but nature is the most common.
A poem must fit a standard length to qualify as a haiku. While most poetic forms allow for flexibility, a haiku must have only three lines. A poem with less than three lines or more than three lines ceases to be a haiku, even if it meets all the other requirements.
A haiku has further rules regarding its syllables. The entire poem must have only 17 syllables, following the 5-7-5 rule. This rule means that the first line can have only 5 syllables. Line two has 7 syllables, and line three has 5 syllables to match the first line. Some modern haiku poets do not adhere to this rule.
Haiku has no set rules for capitalization or punctuation. The poet is free to structure the poem as he sees fit. Most haiku do not rhyme, though the poet is free to use rhyme if he chooses. Haiku poets typically rely on assonance and alliteration, rather than rhyme, to enhance the poem's cadence.