Rose_P

Bearisgray is right. It's a slight error in the illustration. Since the fabric was trimmed after the first picture, it should be up one and over to the right one mark from the bottom and left side of the ruler. But the instructions still make sense if you ignore that little detail and notice that point B and point A have been switched around.

Still, if I can, I much prefer to use a square ruler for trimming if I can find a ruler the size I'm trying to get. It allows me to put the block on a small mat that I can turn around, avoiding the problem of trying to pick anything up and put it back down, lining everything up a second time.

I think I understand what gobucks7 is trying to say: A ruler is usually numbered outward only from one end of each side, so if it is intended to provide a whole number of inches plus the 1/2" for seam allowances, you are going to get a situation where that half inch mark falls on one side but not the other. The only thing the ruler makers could do to make it possible for you to place the half inch on the side you want every time is put an extra set of numbers on all sides. This would be very cluttered and confusing. It's just better to put your "0" start point where you need it to be as you are making the cut, or learn to keep in mind that if you're looking from the 3 1/2" (or whatever and a half) point, then the full inch distance from that corner will be at the 1/2" marks. I have managed to make mistakes where I am thinking in terms of those full inch marks and trim off a half inch more - or if I'm lucky, less - than intended.

This is difficult to explain unless you're actually looking at a typical ruler that is square and includes the extra half inch. There are a few rulers around that are whole numbers of inches, but they're not common. When you use them to trim a block, the finished size of the block will be something and a half inch, which is usually not what you're trying to achieve.