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Most rulers on the market were not convenient for left handed people and can not be recognized clearly on the black paper.
This Color-filter Ruler has a groove in the middle. And scale was printed in opposite turns on each side of the groove which made the ruler can be easily used by both right handed people and left handed people.
The graduate is printed in white on the front and black on the back.During use,the Color-filter Ruler can be easily used in measuring the length not noly on white paper but also on black paper.