Contract

 Warning: This page covers major spoilers regarding the identity or fates of certain characters. Be careful if you aren't up-to-date with the latest chapters. Contract (契約 Keiyaku) is a binding verbal agreement between a Devil and a Human.

Overview
Contracts can only be formed between devils and humans. Thus, fiends are incapable of performing contracts. Contracts are binding in nature and cannot be broken on pain of death. When a devil offers a contract to a human and the human accepts, neither can go back on their word.

By sacrificing something the Devil finds valuable and will cause suffering, humans are able to use the supernatural abilities of contracted devils. Humans can sacrifice their body parts, their senses, their lifespans, their legal children or their legally responsible individuals (for governors etc.) by verbally accepting or offering the terms. They cannot sacrifice someone unrelated to them. However they can incapacitate someone else to offer them to a Devil, as seen when Santa Claus is able to offer an incapacitated Denji by bringing him to Hell with one of her perfect dolls which also indicates that a proxy is adequate to perform legitimate contract.

Devils are able to take their sacrifices by seemingly making them vanish instantly, as seen when the Ghost Devil takes all of Himeno. Sacrifices can also be taken regardless of distance, as seen when the Hell Devil took Santa Claus' beloved children while they were in Germany.

Humans and their contracted devils can communicate with each other by being summoned as seen with Aki Hayakawa and his contracted devils, the Curse Devil and Future Devil, and with Himeno when she talked to the Ghost Devil's hand. They can also learn their remaining lifespans by asking it if thats the cost of the contract.

Devil Hunters tends to hide the details of their contracts and the abilities of their contracted devils to prevent their enemies from learning their weaknesses and strengths, as it is shown by Kobeni Higashiyama. Pingtsi is able to see what Devil contracts a human has by transforming her eel-like hair ponytail of hers into a monocle and looking through it.