Although the outer surface of the roller is smooth and only has numbers, in fact, there is a circle of wheels (some hidden inside the number wheel) on both sides of each digit, and there is a pinion gear meshing with two number wheels next to the adjacent two digits. However, the pinion has a special shape and is usually slippery, and only drives the higher bit to rotate together during carry out. Although the principle is not complex, it is difficult to describe in a single sentence, and it is not easy to describe its actions graphically. The principle will not be explained here.

When using air, you can see the lowest digit wheel slowly rotating. When the lowest bit changes from 0 to 9, the digit wheel in front of it rotates once to increase the reading by a number, which is called rounding. Obviously, this is also a decimal counting device called a "roller counter.". Its appearance is shown in Figure 1.