Seal knob (印纽),  refers to the hardstone carving or small decorative relief at the head and front of a seal. The associated carving technique is called knob carving (纽刻), a traditional technique that originated in ancient China and later spread to other East Asian countries including Japan and Korea.
In ancient China such as during the Zhou, Qin and Han Dynasties, the head or top-side of a seal was named Niu (simplified Chinese: 纽; traditional Chinese: 紐). Later after the Qin-Han period, it's also known as Yin Niu (simplified Chinese: 印纽; traditional Chinese: 印紐), and Yin (印) here stands for seal. In this sense a seal knob could also be called a seal head (yin shou 印首).
Notable is, the character for knob (niǔ) is sometimes written as 钮 in simplified Chinese (with 钅), and 鈕 in traditional Chinese (with 金 ), instead of using the nowadays more commonly used 纽 (with 纟) or 紐 (with 糹) respectively, mainly because in very early period of time, governmental seals were mainly made of metals.
In addition, a seal knob is also referred to as a seal nose (yin bi 印鼻): "the nose of seal"; Bi (鼻) means "nose", perhaps because in ancient time people needed a rope to pull on the seal through its top, just like pulling on an ox or slave through his nose.
Zhou, Qin, Han Dynasties
The head of a Zhou or Qin seal is often a bar, handle/stem, tile, or ring shape. During these periods, seals were normally official and used in government business. The material to make a seal was normally a metal such as bronze, copper, or iron, because metals are durable were considered as "immortal", which could represent the authority of the rule or government. Private seals were not so commonly seen and mainly for those very high ranked officials and nobles, as well as the Emperor. The head of the seal, which was called Niu (鈕/钮), rarely had artistic elements. Its use was just for convenience in handling the seal, such as a ring to hang the seal on their waistbands with a cord.
In the Han Dynasty, the head of seals commonly represented turtles or pyramids, which has a long life, and had propitious meanings, representing the stable, immortal authority of the government. Jade was often used.
Tang, Song Dynasties
During the Tang and Song dynasties, governmental seals were still the most dominant, and their style continued those of the Han Dynasty. But in the Song Dynasty, especially the Southern Song Dynasty, due to the rise of artist groups and scholars, and the prosperity of the economy, common people also needed to express or identify themselves, and private seals became more and more popular.
Yuan, Ming, Qing Dynasties
In the Late Yuan Dynasty, some famous specialized seal artists or craftsmen appeared. In the Mid and Late Ming Dynasty, seal sculpture became truly popular among artists and scholars for the first time. Together with the Kang-Yong-Qian period of Qing Dynasty, these two periods are considered as the golden periods of seal sculpture. Seals, especially the emperor's seal, featured a Chinese dragon or a Qilin, instead of a turtle. The so-called dragon-turtle is also seen.
The techniques to make a seal sculpture the same as for sculpture in other hardstone carvings, though the area to be carved is very small, requiring special skills and tools.
For many private seals, such as the seals of artists, calligraphists, painters, a wide range of subjects are depicted. And the seal sculpture of their seals (such as so-called Xian Zhang (閑章/闲章; roughly translated as "seal of leisure")) can be pets, landscapes, and so on. It could be closer to their daily life or rusticity.
Sometimes, a seal stone has different colours on its surfaces or inside, and the sculptor needs to make a perfect combination of these colours with his/her sculpture, in ancient Chinese, this is called Qiao Diao (巧雕; roughly translated as "sculpture of cleverness").
|Wikimedia Commons has media related to Seal knobs.|
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- Seal script, the Chinese character script created during the development of the Chinese seal art