Abstract: A deadlock of the customary type for locking and unlocking a dead bolt is equipped with a conventional cylinder having a cylinder plug rotating within it, the cylinder plug carrying pin chambers aligned with pin chambers in the cylinder for reception of pin tumblers when shifted in position by cut irregularities on the edge of the key. With chambers in alignment an appropriate key inserted in the keyway shifts the pin tumblers to positions coincident with a shear line to permit rotation of the plug to either locked or unlocked position. Unless the chambers are in alignment, the tumblers cannot shift and the key can be neither inserted nor withdrawn. For preventing withdrawal of the key on, for example, the inside of the door, when the dead bolt is locked, there is a detained key mechanism.