I'm sure I'm not the only one who has noticed this, but I discovered this on my own, and I'll take whatever credit folks want to toss my way.
Here's a handy way to make a ruler with nothing more than an 8.5x11 inch piece of paper. Holding it as a portrait (taller than it is wide) fold it on a diagonal line so that the bottom edge lines up with the left side (or the right side, if you are so inclined). When folded, you'll have a skinny rectangle at the top which you could cut off, leaving you with a piece of paper 8.5 inches square.
The long diagonal of an 8.5 inch square happens to be just two hundredths of an inch longer than 12 inches. Close enough for government work, as they used to say down on the farm.
Fold that line in half, and you have the 6 inch mark. Fold in quarters for the 3 inch and 9 inch marks. I'll leave it to you figure out how to get the rest.