This law also does not specify which finger to use. The fingerprint of the hand used to sign a contract is the thumb fingerprint of the right hand. Fingerprints are raised lines on the epidermis. Because human fingerprints work together with the environment, although everyone has fingerprints, they are all different. Hold out your hand and observe carefully, you can find that there are several types of small fingerprints: there are concentric circles or spiral lines, which look like swirls in the water, called bucket-shaped lines; Like a dustpan, they are called -shaped patterns; some patterns are like bows, called bow-line patterns. In addition to the shape of each person's fingerprint, the number and length of the patterns are also different. It is said that two people with identical fingerprints have not been found. Fingerprints begin to appear in the third and fourth months of the fetus, and they form in about six months. When the baby grows up, the fingerprint is only enlarged and thickened, and its pattern does not change.