Sorry, somehow I missed this post completely. :oops:
The lines on cutting mats are not accurate. Some are better than others but almost all of them seem to have at least one area that is off. What you can do to check your mat is to take one of your rulers and simply lay it over your mat and try to match up the lines. You may notice where the lines on the mat drift from one side or the other compared to the lines on your ruler. I used to use the lines on my mat. I would be super careful to measure and cut everything out just right but there were times my pieces wouldn't sew together right and I used to get so frustrated! It wasn't until I took an Intermediate Piecing class that I learned mats are not printed accurately. I measured my matt and was stunned when the differences added up to about 2 inches, with one area between the 16-18 inch area on my mat really being off. :shock: It wasn't easy but I forced myself to use only my ruler for measuring and I have to tell you, my piecing has become very accurate since then. Yay! I will typically use one line on the matt to line up my fabric, but otherwise I measure everything with my ruler. It does make it easier when flipping a used mat over and using the blank back. I simply draw a horizontal and vertical line with a permanent marker and I'm good to go. :thumbup:
~Tiffany