There are approximately 200 of them in circulation in the Shelf. Identical 15-centimeter blades, single-edged, no manufacturer's mark, no serial number, no identifying feature of any kind. The alloy is unknown — metallurgical analysis returns contradictory results depending on the lab, the equipment, and apparently the mood of the analyst. The blades do not dull. They do not rust. They do not chip, bend, or break.

They appear. In drawers. In pockets. Under pillows. In the lining of a coat you've owned for years. No one has ever witnessed one materializing. No one has ever purchased one. They are never found by anyone who does not need one — and the definition of 'need' appears to be the city's, not the carrier's. A woman who had never held a knife found one in her apron pocket the morning her ex-husband was released from detention. A teenager found one in his schoolbag the week a gang began recruiting on his block. A grandmother found one in her knitting basket and laughed and said, "I was wondering when you'd get around to it."

The knives cannot be sold. Every attempt to transfer one for money has resulted in the blade disappearing from the buyer within 24 hours and reappearing somewhere else — presumably wherever the city decides it is needed next. They can be given freely, but only if the giver genuinely does not want payment.

"The city decided you needed protecting." This is what Shelf residents say when someone finds an Unmarked Knife. They say it the way you'd say the sun came up. It is simply what happened.