Two-person rule

Per US Air Force Instruction (AFI) 91-104, "the two-person concept" is designed to prevent accidental or malicious launch of nuclear weapons by a single individual.

Nobody on board has the combination to open these safes; the unlock key comes as a part of the launch order (Emergency Action Message) from the higher authority.

[3] Notably, Major Harold Hering was discharged from the Air Force for asking the question, "How can I know that an order I receive to launch my missiles came from a sane President?

[4] Two-person integrity (TPI) is the security measure taken to prevent single-person access to COMSEC keying material and cryptographic manuals.

In the United States Air Force (USAF) policy concerning critical weapons, a no-lone zone is an area for which entry by a single unaccompanied individual is prohibited.

As another example, firefighters operating in a hazardous environment (i.e., interior structure fire, HAZMAT zone, also known as IDLH, or "immediately dangerous to life or health") function as a team of at least two personnel.

As an extension of the broader rationale for the two-person rule, regulations for some companies or not-for-profit organizations may require signatures of two executives on checks.

Some software systems enforce a two-person rule whereby certain actions (for example, funds wire transfers) can only take place if approved by two authorized users.

While such requirements are common in financial systems, they are also used in controls for critical infrastructure, such as nuclear reactors for electrical power generation, and dangerous operations, such as biohazard research facilities.