Ditto - and they are especially useful when making patchwork jackets and clothing.

I alwaysw use them when I cut strips and then squares and sew half triangles. I make sure the mat and rulers are the same company.

I always use the same mat and ruler to keep measuring consistent

When the first mats came out, they didn't have the lines, so that is how I started cutting, and how I still cut. I was told that the lines on the board aren't 100% accurate.

I have never used the lines on my mat.

I always do it that way, possibly because I am self taught? I was reading some older threads earlier today (when I should have been working <g>) and I saw that lots of folks hee don't use the lines on the mat at all. Not sure how they do that.

The issue of accuracy aside, my main objection to teaching someone how to cut using the lines is one of fundamentals.
If you are in a situation where you don't have a mat with lines
(workshop, class, retreat, at a friend's, etc.) can you cut without the lines? I have a friend who can't, and sometimes it's a problem when we get together and she has to use a mat without lines...she just can't do it. But then, I'm a dinosaur and I think beginners should at least have a rudimentary knowledge of drafting patterns and making templates. Okay, you can get your guns out to shoot me...

I agree with crashnquilt. You can use one or the other but not both. Same with sewing machines.

That's my way of cutting too.

I use the lines on the mat and have had no problems, thought that was what they were for.