Haiku is by definition 3 lines. Five syllables in the first, seven in the second, and five in the third. Haiku requires some reference to a season or seasons or to seasonality & in Japanese one has 'seasonal words'. Words that bring senses of the season - only felt during that season ... "cicada" for example means summer, "cool change" & "mangoes" also summer... "chestnuts" are autumn... & "camelias", late winter.

One difficulty is that haiku's 5-7-5 syllables when read out in English have intonational differences compared to a reading in Japanese, which is almost always consonant vowel/consonant vowel combination, or a single vowel. There are certain intertexuality and styles of writing that can only be done in Japanese.

In the west, verse often called Haiku are in fact Senryu. Devised during the Edo period, Senryu follows the 5-7-5 syllable system similar to that of Haiku, however, Haiku uses seasons and feelings, while Senryu uses wit and is often a satire.

  Five begin haiku;              
  Seventeen in the whole thing,  
  Seven in line two.             
David Edwards : <de3@ucsd.edu>