Razor Knife
AKA: Stanley Knife, Carpet Knife, Section Knife, Box Cutter, Utility Knife
Blades are brittle and extremely sharp. Use hand and eye protection. Be extra careful of finger tips when cutting close with a ruler. Do not apply more pressure than necessary, as the blades are designed to snap off when blunt.
A small razor blade knife for cutting carpet and other textiles and similar materials. This knife usually has a sectioned blade which allows you to snap off broken and blunt sections to reveal new, razor-sharp sections.
Changing a blade should be done with hand and eye protection, due to the likelihood of snapping blades flying off unpredictably. Use pliers to grip the blade at the section line (you'll see the line on the blade) and snap sideways.
If your knife has replaceable blades instead of snap-off blades, you may need to replace them by unscrewing the body of the knife and swapping out the entire blade, or by pulling a clip or lever to release the blade. The method will depend on the design of your knife.
One type of razor knife.
A snap-off blade, showing the snapping points.