why does Jenny do this?
 

I was taught and I am sure many others were too, that you never use the marks on the mat to cut your pieces, always use your ruler. Well I have watched several of Jenny's tutorials (MSQ) and sometimes not always but sometimes she uses the marks on the mat. Don't get me wrong I love her and all she does for quilters, but why does she do this? One of the things I love about her is that she does not intimidate new quilters.

Why not use the markings on the cutting mats? Why would the markings be there if we can't use them? I've checked mine and the markings appear to be same as the ones on my ruler. I use both when cutting.

I think a better question is - why not?

I use the mat lines sometimes, the ruler other times. Depends on what I'm doing and whichever works best.

Some people have said their mats are inaccurate; mine seems to be just fine. So why not use those lines, if that's what works and is most convenient?

The reason is that not all mats are accurate in their measurments. I think it'd depend on what you were cutting, some things like HST takes very accurate measurments and others not so much. If I'm doing a large square for emb, etc then I'll use my lines on my mat. I always use them to make sure my fabric is straight before I use a ruler to cut it. It's more important to use the same rulers thru out a whole quilt, etc because not all rulers are the same and you can have problems using different ones in a project.

I use the lines on the mat when I'm cutting something like a pillowcase.

I use my 1/8 inch grid ruler when I'm cutting tiny squares.

An inch is an inch, no matter where it's measured. I think that the "difference" between rulers is actually due to the difference in how we are able to perceive the markings, not on whether the markings are accurate. Rulers have thickness, and the lines etched into them should be accurate on the top, or on the bottom, but not on a slant from bottom to top. (Sorry, that's hard to explain.) Many times I think it is more accurate to use the mat, whose surface is two-dimensional, rather than the rulers, which are 3-D. But it's not always as convenient to use the mat, since it doesn't have as many lines and it means moving the fabric more.

I will use both, as the lines on my ruler line up perfectly with my mat - easy enough to see if they are the same.

She probably does it for expediency's sake. The recommendation that's always made the most sense to me is to be consistent with the tools you use to measure and cut, which means using the same brand of ruler throughout a single project. A mat falls short because it's difficult to cut things in increments of an inch, so I ignore the lines on it. In fact, I often turn my mat over to cut on the non-gridded side because it's less distracting.

I'm glad to see that others use the lines on their mats. I've always used mine. Measured them when I first heard that "rule" and they measured true to size. I thought I was committing some mortal quilting sin LOL!

this is what i do, also, as my mat's markings are accurate