Necklaces, also called collars or dog collars, were neck shackles for prisoners on a death row in Deadman Wonderland prison. The necklaces were used for tagging death row inmates and also have tracking system installed inside of them which can track a prisoner if they try to escape. They also carry a lethal poison inside of them that is injected into the carriers jugular veins, which will kill them within 3 days. Unlike regular prisons, death row inmates can purchase temporary antidote that is in a shape of a candy using CP. The necklace also has a timer which has a countdown from 72 hours since the last antidote intake. This data can be accessed by the prison warden. When the prisoner is near death, a red light on the necklace will start blinking and beeping, warning the prisoner that they're near death. When the countdown stops, the necklace will display "DEAD" and kill the prisoner. After that, it will detach itself and display "UNLOCK". The necklace can also be unlocked by a special key. If the prisoner does not take the medicine every 3 days the body will stop functioning and it will result in a painful death.
When the truth about Deadman Wonderland was discovered, all prisoners were sent to regular prisons, with the exception of some having new trials and having their necklaces removed.