i've been cutting lots of triangles for my "indian blanket" quilt. the fons & porter 60` ruler works really well for this. i especially like two things about it: the top tip is blunted and it has maths on the ruler for you. the ruler is marked with the strip size you need for the finished height you want to achieve.
all that said, i was having one trimming issue - whenever i reached the end of the strip, it was hard to figure out where to cut the last piece straight so i ended up with a half piece, which i need for the quilt. after all the triangles are cut, you are left with a bit that is angled on one side but too wide to be a half triangle.
what i wanted was a piece that is cut down that middle dotted line there, bisecting the triangle in half.
here's a closer look at the line i wanted to trim on. trouble is, of course, that you can't cut through the ruler. so i usually either flip the piece over, shift the ruler to the side, and trim off on the angle (which leaves you with a skinny, angled piece with two bias cut sides), or i use the straight ruler to make a cut, but this is hard to figure out accurately.
well, the other day, my long-buried geometry knowledge kicked in and somehow i got the idea to simply shift the ruler around to get my straight cut. please don't ask me which theorem i used or the measurements of the angles that make this work. i don't know. but i do know this works!
as you can see, i lined up the center line with my outside angled edge and my blunt top tip with the top of the fabric. then i cut down the angled edge of the ruler.
this gave me the perfect half triangle i was looking for and the leftover piece is straight cut, not biased, on both sides.