Here it is:
How to cut a large square into half-square triangles using a rotary cutter if the longest ruler you have around happens to be smaller than the diagonal of that square. (Hint: Moving the ruler up and down the path is the wrong way of doing it.)
So here you are, with a ruler and a large square piece of fabric.
As it turns out (of course!) the ruler is too small to do the job properly.
Here is what you do. Fold the piece diagonally (shown on the left). Press hard along the fold line and unfold (shown on the right).
Fold the piece along the other diagonal, perpendicular to the original fold line (which is illustrated as a black and white strip here).
The folded square can be approached as a smaller triangle with two layers of fabric and a fold line (the first one) indicating the place to cut. The ruler should be long enough to cover it completely.
Cut along the line and unfold both resulting pieces. You are done.
As I said, it's simple, but I believe it is worth knowing.