the special part about it is that it has a recessed channel that locks over the seam, holding it in place while you trim.
this picture shows the wring side of the unit, so you can see how the seam just nestles into the groove:
it doesnt matter whether the unit is right side up, or right side down,
however, we also have them for Flying Geese units, and Half -square Rectangle units,( which actually have two rulers in the package,)
I really like the ruler, although it is a bit pricy at 18.50, but it sure does work well,