Ruiju-Karin is a collection of poetry. Shoshizoshocho in Shosoin Monjo (document collection of the Nara period kept in Shosoin) states that seven volumes of Karin came into the collection on July 4, 751. If 'Karin' in this document refers to "Ruiju-karin," it means that this poetry collection consisted of seven volumes. It is assumed that Ruiju-karin was written or edited by YAMANOUE no Okura. The date of compilation is unknown, probably in the early Nara Period. The title can be found in documents prior to the Kamakura Period, such as "Masako Naishinno Eawase" (Poetry matching held by Imperial Princess Masako), "Waka genzaisho mokuroku" (A list of extant books of waka poetry), and "Fukurozoshi" (Notes in a Folder, the collection of poetic lore). Ruiju-karin is not extant. "Waka genzaisho mokuroku" indicates that 'Ruiju-karin was in the treasure house of Byodoin Temple,' so it is a proof that the poetry collection was still extant until the end of the Heian Period.
Some parts of Ruiju-karin were cited in nine instances in Volumes One, Two, and Nine of "Manyoshu" (the first major anthology of early Japanese poetry).
It seemed to be a collection of poetry prior to the days of Emperor Monmu with descriptions of the poets and backgrounds of poems, and integrated poems with similar themes into categories. The term "Ruiju" indicates gathering poetries of similar categories. "Waka genzaisho mokuroku" also indicates that 'Okura gathered old news to compile the Karin collection', thus he might have made historical study on poetry.