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with 9" squares if you are off a thread or two it won't matter.
Just make sure you measure the lines on the mat and always line up on the same side of the line on the mat
I'm confused about whether you want a 9" unfinished block, which gives you an 8.5" sewn/pieced block, or a 9.5 inch unfinished block that gives you a 9" sewn/pieced block.
I only bring this up because I've measured twice with the wrong measurement (used the finished size of the block to cut, not the unsewn size) and therefore cut wrong...a painful, expensive mistake!
The advice you've been given by others is good. Now you just have to figure out which method will work for you.
One hint - if you're cutting folded fabric, press the folds with your iron. Can't remember where I learned that trick, but it seems to minimize 'elbows' made when cutting through the fold.
Good luck on your quilt!
I bought a 24" square ruller for just that reason. I sometimes have to cut blocks/strips over 6 1/2" and this ruler allows me to do that easily with out having to put 2 rulers together. I got it at Joann's with %off coupon so price was not too bad. Well worth the money Also I have a 12" square ruller that I think I bought as a set(always at joann's with % off coupon). I do not use my mat lines for accurate cutting.
"In the crazy quilt of life, I'm glad you are in my block of friends."
Go to your local glass company and ask them to cut you a plexiglass square the size you want. Tell them it has to be exact because of what you are going to do with it.
Or, use the 12 inch square you probably already have. Fiddle with it long enough and you'll see that it has nine inch lines all the way around and you can lay it on your fabric to get any size you want that doesn't exceed the size of the ruler. froggyintexas
I often cut larger strips of fabric using my 12.5" square ruler. I would cut the 9.5" strips, then subcut into 9.5" squares. I also use my 12.5" square when cutting multiple strips, like 3.5", I would cut at 10.5", 7" and 3.5". I had seen a quilt teacher on TV cut this way - after straightening the end, I would subcut the strips and never move the ruler, makes for more accurate cuts this way. I know this may be confusing, but use the method that works best for you. I will say that I seldom use the cutting mat to cut squares or strips, but that's just me.
I never use the mat as a guide. I learned early on that the fabric can move and my cuts might not be accurate. With the ruler holding the fabric, it will be more accurate. It is not hard to use two rulers.
There are many ways to cut strips, just find the one that works best with the tools you have. I generally don't use the mat as my guide, although I have checked the grid and it is accurate. I was taught that the reason we shouldn't use the mat in this way is because by cutting on the lines, we would tend to cut in the same places all the time, and this leads to premature wear on the mat, causing grooves or slivering of the mat surface.
I have the June Tailor shape cut ruler and like it a lot. Just be absolutely sure you are in the right slot before making your cut. Learned that the hard way.
Here is a video from Marci Baker, the first several minutes of which shows how she cuts strips. Her mat has no lines on one side and she prefers to use that for less "visual noise". Her method of establishing a straight edge is one I had not seen before.