I have rulers from 4 different brands, and I lined them up side by side, and they are all exactly in agreement about the length of an inch. They also line up perfectly with my Olfa cutting mats. The advice to buy all the same brand may have originated with one of the earlier companies hoping to discourage us from buying competitor's stuff. Or maybe there are some quilters out there who are OCD enough that they're using microscopes to check for accuracy. I'll admit I'm not one of them. One word of caution: If rulers are different thicknesses, when you look at them from an angle, there may be a tendency to see the lines differently because the light is distorted, sort of like looking through a glass measuring cup at an angle.