I've heard over and over that rulers can be different sizes, and not to use your mat, etc. But... I have checked my rulers, and they all agree that an inch is an inch, and my mat agrees with the rulers. I think the differences come about because of the different way the lines are marked, the coloration of the rulers, and other factors. I know I can see certain fabrics better using clear rulers, and for others the greenish rulers work better. Sometimes the thickness of the ruler interferes with the cut, because the lines are nearer the top of the ruler. The only "mat" I have found to be really off in the measurement was the June Taylor ironing station, which had a grid marked on top of the ironing surface, and that was WAY off.