Renga is linked verse: stanzas by different people. The classical tanka or waka is five lines, with the following syllable count: 5/7/5/7/7 - the first in that form appear in the Man'yoshu. The first three lines were often distinct from the last two. In renga, one person would write the first three lines, then someone else the next two, and 3- and 2-line verses would continue to alternate, by as many poets as were involved. Any two adjacent verses had to be readable as a standalone poem - indeed, often two 5-liners sharing a verse would have completely different subjects, by differing metaphoric interpretations of the intersection lines. Donald Keene points out that this is easier in Japanese, where the language is ambiguous enough to offer lots of wordplay, and pronouns are very loose where they exist at all. These sequences of verses could go on at great length, some lasting for thousands of lines. There were often rules: how often certain words should be used, themes and so on.
The first collection was in 1356. Waka remained the great love of the aristocracy, but renga was more popular with others. The first verse was generally written in advance of a renga session, by the most noted poet. This 5/7/5 stanza was therefore of crucial importance, and was called the hokken: it isn't hard to see how this eventually acquired an independent existence of its own, more of which in the haiku section here. It's worth highlighting the most revered renga master:
forwards: Buddhist Poetry