If two or more airplanes fly together, it is said they are flying “in formation” and need to be skilled in that sort of endeavor. It is usually the military aviators and/or their flight teams who do this, and they are highly trained and are (usually) flying similar airplanes.

When two planes fly in close proximity, it is formally known as formation flight, which is defined by the FAA as two or more aircraft that, by prior arrangement between the pilots, operate as a single unit with regard to navigation and position reporting. In a "Standard Formation," the wingman stays within one mile laterally and 100 feet vertically of the flight leader. In a military context, this is often done for mutual defense, tactical efficiency, or training purposes, allowing the leader to focus on navigation while the wingman monitors the surroundings. For civilian aircraft, you might see this during airshows, commemorative "flypasts," or when specialized photographers are performing "air-to-air" photo missions. If you see a smaller plane flying very close to a larger commercial-looking jet, it might be a chase plane performing a test flight or a military intercept for a localized security violation. Regardless of the reason, the pilots must communicate constantly, as the responsibility for maintaining safe separation shifts from Air Traffic Control to the pilots themselves during the formation.