It was a progressive poem, where one person would write the first three lines with a 5-7-5 structure, and the next person would add to it a section with a 7-7 structure.
The chain would continue in this fashion. So if you wanted some old examples of haiku poems, you could read the first verse of a “tanka” from the 9th century.
The first verse was called a “hokku” and set the mood for the rest of the verses. Sometimes there were hundreds of verses and authors of the “hokku” were often admired for their skill. In the 19th century, the “hokku” took on a life of its own and began to be written and read as an individual poem. The word “haiku” is derived from “hokku.”
A haiku poem consists of three lines, with the first and last line having 5 moras, and the middle line having 7.
A mora is like a syllable but it is merely a sound unit. The two are not actually identical. Since the moras do not translate well into English, it has been adapted and syllables are used as moras.