The traditional hokku usually was written in six verses or more or less 5, 7, 5 syllables and they had to follow ("on-ji"). The Japanese word "cow", meaning "sound", corresponds to a "mora", a phonetic unit similar but not identical to the syllable of a language such as English. A haiku has a special "season word" (the "kigo") to represent the season in which the poem is set, or a reference to the natural world.