When I place the mat on the base, I can begin squaring up a piece just like normal. Cut the left side and top and clean that up.
To square up the other two sides, I don't need to move the fabric and the ruler. I rotate the board. Now I'd turn it 180 degrees, but then you wouldn't see that I was turning it.
Trimmed up, (you can see the mat is upside down from the earlier photos), I can line up along a number of angles and cut the piece up even more.
Easy-peasy. This is a good thing.
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