Yuin is a seal that has no character belonging to any individuals or corporations such as full name, gago (pseudonym), trade name and yago (shop name), but has inmon (words or symbols engraved on the seal) curved. It is also called gokuin (another name of yuin, seal carved favorite words or phrases). The words for the seal are often extracted from words or phrases that express literature or thought, which tenkokuka artists (carver using a special Chinese character, tensho) prefer to use.
In addition, the seal which is antagonistic to yuin is called kosoin.
Yuin' is the name of the seals indicating okyakuin or akkakuin (seal on the lower side of documents) as opposed to the inshuin (kanbo) among the seals affixed over two edges on the right shoulders of documents in order to prevent forgery of the emperor's written appointments. Yuin came to mean the seals in the sense that it could be impressed anywhere on the documents when it began to be used on the calligraphy and pictures.
However, if you try to grab the meaning of inmon, yuin dates back to the seals called kansho in the Qin and Han Dynasties of the Warring States Period (China).
All the kansho seals did not have any practical purposes but were private seals. As lucky words and phrases or words of warning were carved on the seals, it is called kitsugoin (seal engraved auspicious words or phrases). Wearing the seal on waist, they tried to invite lucky omens. In addition to the seals, there are also seimeiin (seal engraved artist's name) with kitsugoin attached.
In Song Dynasty and later when tenkoku (seal carving using special Chinese characters, Tensho) by men of literature became popular, cherished motto, Chinese poetry, religious phrases and refined sentences came to be used as inmon. Such seals are called seigoin or yozeiin. The seals were put on calligraphy or pictures of silk or paper as ornaments. It is said that the seal of '賢者而後楽此' was the first seigoin. In Yuan Dynasty, the seals appeared in the works of Mengfu ZHAO and Miang WANG, and in Ming Dynasty a huge number of seigoin were used in and after the works Wen PENG and He ZHENG.
When it is used as a rakkan (signature and seal) in calligraphies and pictures, it is common that yuin in the form of inkoku (carving to remove the pattern) in red ink is put under the full name of the artist in hakubun (carving to remain the pattern). The custom was started by Shen ZHOU in Ming Dynasty, and later was spread by Go school.
In Japan, yuin came to be made by the calligraphers and men of literature influenced by "Hikodo inpu" reprinted in early Edo period.
With the appearance of yuin, the artistry of tenkoku became recognized. In Japan the works of tenkoku are presented at calligraphy exhibitions.