If you've ever cut front surface mirror, you know how difficult it can be to slide a razor blade through the blue nitto tape protecting the mirror.   These tissue blades are super thin, used by medical and scientific professionals.  These blades fit incredibly easy through mirror with nitto tape, even when 2 pieces are folded so the tape is 'back to back'.  I've used these for years - they're great to have when cutting mirror.
 
Also the perfect blade for polymer clay and any other crafts demanding super thin cuts.  The price is per 1 blade.