Binary system is also called "binary coded decimal system" or bcd code for short. A counting method of representing decimal numbers in binary code form. A decimal number is represented by four binary numbers. There are many representations, among which 842 1 code is commonly used, and from left to right, 8, 4, 2 and 1 are used to represent the bit weight of four-digit binary numbers respectively. For example, the decimal number 97 can be expressed as:1001011. This counting method has been used by American Indians; Greenlanders also have traces of this system: 20 uses "one person" and 40 uses "two people".