In a traditional Hay(na)ku, there are: A tercet: 3 lines. A total of 6 words: 1 in the first line, 2 in the second line, and 3 in the third line. There is no restriction on syllables or stressed or rhymes.
Variations:
In the 'reverse' haynaku, the longest line is placed first and the shortest last. The total is still 6 words: 3 in the first line, 2 in the second line, and 1 in the third line. Multiple hay(na)ku can be chained to form a longer poem. There are other variations also.