Hi,
I am a newbie to the message board, and I am using you to get my feet wet (so to speak). I don't know where to get that tool, but I have a better description of it, in case anyone else does. The show was "Sewing with Nancy" and she was making a half triangle square with two squares. The tool's cut outs allowed you to line up the two opposite points and once thy were lined up you just marked the wrong side of the top fabric square on either side of this "ruler."
After removing the tool you then sewed on the lines. Using your rotary cutter down the center of the two lines, there was a perfect seam allowance and once ironed open, two perfect squares of two half triangles. I am sure, if someone had the e-mail address of the "Sewing with Nancy" show, they could tell you how to get one. It was neat, and I want one, too. If I find it, I will writew back. :-)
Isadean