..I'm doing a complete reversal of opinion on these because I'd only been trimming 1 or 2 layers of fabric and that was a problem even to get through both layers without having to run the blade 2-3 times!
So I put in a brand new blade - Olfa brand - to use with the bigger 24x6 ruler. Tried to cut 4 layers for some strips. Even after 6 passes it was still not cut through! When I did cut through, the edge was ragged and shredded! After trying one more time found the blade was, as another member said, completely destroyed. The measurements are not accurate! Tried for a 3" strip and then turned the ruler sideways and found the strip was 1/32" over that!
There are no special directions on the packaging. The inset blade is at an angle so it sticks out farther at the bottom. Arggghhhhh!! Total frustration! Certainly I will now contact the company.
I do like the markings and find they are uncluttered. I'll just use the side without the sharpener because of this.