Because one or two of the other four fingers have been cut off, you can grasp things, but without your thumb, it will be difficult to handle things. In ancient times, some people would cut off their index finger to avoid guilt, so it was later stipulated that they would bet with their thumbs. In this way, if he wants to escape guilt, he will lose a hand by cutting off his thumb, and he might as well take the blame.

But in modern times, it is usually the index finger, which is more convenient.

When interrogating prisoners in prison, it is common to make a bet with your thumb.

Sign a contract casually, with more index fingers and less thumb risk.

I hope my answer to you can help you, and I wish you all the best! ! !