Believe it or not, I found that my cutting mat measurements were off!! Not too long ago I started using a 36x24 inch Fiskars cutting board, and was using the inch marks to line up my fabric to make cuts. Well, when my blocks were screwy--which has never happened before, I finally measured my ruler against the cutting mat--and found that the cutting mat was off 1/4 inch by the time the mat reached 18 inches!! I measured my cutting ruler against other rulers, and the ruler was spot-on in its measurements. It was the mat that was not accurate!! Grrr. So now I don't use the mat to make cutting measurements, I just use it as a mat.