For a long time in ancient China, due to the low literacy rate of ancient Chinese, the ancient Chinese used "pictorial" as their signature on contracts. Then you may have a question: Isn't it easy to draw a circle on paper? Why did the ancients use this method as their signature? Aren't they afraid of being used by others?
This starts with the history of signatures and paintings. According to ancient historical records, the earliest signing of paintings was in the Tang Dynasty. At that time, if you left your own personal notes on documents, calligraphy, paintings or contracts, it was called a "pledge", which is equivalent to the current "signature" and "signature".
At that time, except for the souvenirs presented to the emperor, which prohibited the use of cursive scripts, other contracts and documents generally used various fancy cursive scripts, and these signatures were collectively referred to as "monograms." In the Song Dynasty, when officials of the Song Dynasty wrote official documents, they usually only wrote their own names at the end of the official documents, so it was also called "affix". For example, Wang Anshi, the famous prime minister of the Song Dynasty, wrote the word "石" in his signature, leaving only one and one, and the word "口" was omitted directly below. Later, some officials thought his signature looked like the word "reverse", so he added a circle in the middle to replace the word "mouth".
So the question is, most people at that time were illiterate and what should they do if they needed to sign? Many people have ideas. They asked the people to imitate Wang Anshi directly and draw a circle instead. This is the origin of the so-called "monogram". In fact, when a photo is taken, the government often requires people to have their fingerprints pressed as secondary evidence, so the image itself may just be symbolic.
On the other hand, since it is the first time for many people to come into contact with a paintbrush, it is difficult to draw a complete circle, so the patterns they draw have certain particularities. For example, Ah Q wanted to draw a circle, but it turned out to be in the shape of a melon seed. Therefore, actually drawing the letter itself has special significance for identifying ordinary people.