Originally Posted by Prism99

Matching selvedges actually has nothing to do with cutting straight strips. Straight strips depend on the fold line being exactly 90 degrees to the cutting ruler. To ensure this, I use a large ruler on the fold line, matching a line on the ruler exactly to the fold. I then butt my cutting ruler against the first ruler. (The edge of the large ruler will be exactly 90 degrees from the lines on the large ruler.) Any variation away from 90 degrees will cause a bow at the fold line.