The X-TraSafe Cartridge Knife was developed as a safe alternative to replacement blades used with standard utility knives. It uses a spring activated blade guard to protect users by closing after each cut. These replacement blade cartridges are for use with X-traSafe Cartridge Knife Model CSI (not included). Knife Type: Utility; Blade Type: Fixed; Blade Material: Stainless Steel; Applicable Material: Cardboard; Tape.