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The two-man rule is a control mechanism designed to prevent accidental or malicious launch of nuclear weapons by a single individual. In the case of a missile silo command crew, both operators must agree that the launch order is valid, and must each complete a set of tasks independently and in proper order to launch. On a submarine, both the commanding officer and executive officer must agree that the order to launch is valid, and then mutually authorize the launch with their operations personnel. Usually, the two-man rule is also backed up with hardware and software measures including command code verification and command keys.

The term used in other safety critical applications where the presence of two people is required before a potentially hazardous operation can be performed. Some security protocols designate areas where human occupation is only permitted when two or more authorized individuals are present; such areas are sometimes known as no lone zones. This is common safety practice in, e.g., laboratories and machine shops. Some software systems enforce a "two man rule" whereby certain actions (for example, money wire transfers) can only be effected if approved by two authorized users.

In popular culture, the Two-man rule is evidenced in the television series Star Trek: The Next Generation, as it requires the authorization of both the Captain and the Executive Officer to initiate the self-destruct mechanism of the Starship Enterprise.

The requirement for the presence of at least two staff members is also used in other contexts where there are security concerns, such as in banks. When bank employees perform some tasks, such as removing money from safes or opening Automated Teller Machine deposits, company policies often require that at least two employees be present.

As an extension of the broader rationale for the "two man rule", regulations for some companies or not-for-profit organizations may require signatures of two executives on checks. These rules make it harder for an individual acting alone to defraud the organization.

• Buddy system
• Dual key
• Fail-deadly
• The Hunt for Red October
• The Sum of All Fears

• U.S. National Park Service article on Minuteman missile launch control centers, with details on operation
• U.S. DOD nuclear weapons recovery manual with reference to two man rule