A no-lone zone is an area that must be staffed by two or more qualified or cleared individuals. Each individual must be within visual contact with each other and in visual contact with the critical component that requires a no-lone-zone area designation. A no-lone zone may contain a cryptographic component, weapon system hardware under test, a nuclear weapon or active nuclear weapon controls.
In the USAF concerning critical weapons, it is a zone in which the presence of a single individual is prohibited. The two-person concept (or policy) is in effect in which two individuals, knowledgeable of the task to be performed, and capable of detecting an incorrect or unauthorized procedure on the part of the other in reference to the task being performed.