The number axis has been introduced into textbooks since the first grade. Considering the age characteristics of the students in Grade One, the first volume of Grade One has similar physical drawings, and the second volume of Grade Two has similar "ruler" lines. In fact, the number axis is the "prototype" of the number axis. Using the number line, we can not only know the order and comparative size of numbers, but also calculate: "Decrement" is equivalent to addition, and "Decrement" is equivalent to subtraction. Therefore, digital lines are of great value.