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We often come across this situation in the process of drawing: When the ink of pen is too much, the escess ink will infilter below the ruler along the edge of the ruler and pollute the paper. It happens because of capillarity.This design is to provent this kind of phenomenon occurred. The front of ruler is protuberant and produce a certain distance from the paper.At this time, it will not infilter below the ruler when the ink is to much because of the capillarity.
Zhang Zitian Li Jian