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This is a ruler. It' s principle is very simple that the two wheals are relative fixed. And it will do the rectilinear motion with the friction between the paper. So you can draw a straight line on the paper when you put the pen on it. Besides, you can get the length of line by the calibration. It has the advantage of reducing the size of the traditional ruler, making it more convenient to carry and use, and can extend straight unlimitedly.