Most Americans are familiar with the presidential jet commonly referred to as Air Force One, a specially modified Boeing 747 with an iconic blue and white paint scheme that carries the president around the country and the world.

Less well-known is the fact that the president is typically followed by a smaller jet that, in case of an emergency, can fill in as an Air Force One replacement, per a report.

That smaller jet, if used to transport the president during an emergency, would also then be known as Air Force One, as that designation follows the president and is applied to any fixed-wing aircraft they may use -- Marine One being the designation for any helicopter or rotary aircraft used by the president.

Air Force One

The aircraft commonly called Air Force One is known in military parlance as a VC-25, one of two specially modified Boeing 747 passenger jets that are outfitted with advanced communications and electronics equipment and other necessary accommodations for the nation's chief executive and those who travel with them.

The VC-25s, which are stationed at Joint Base Andrews in Maryland near Washington D.C., have been in service since 1990 and feature a host of other special performance capabilities not available on the civilian versions of the aircraft.

In most circumstances, the VC-25s are capable of transporting the president and their entourage wherever they need to go, but there are a few situations in which that isn't always feasible, and for those exceedingly rare occasions the Air Force has a backup plan.

The president's "secret" Air Force fleet

In 2017, Politico reported on the president's so-called "secret Air Force," which is a fleet of aircraft intended to ensure the survivability of the president and the continuity of government in case of a major emergency, such as an enemy attack or major natural disaster.

Not all parts of that fleet are a secret, such as the Boeing 757s that typically ferry around the vice president, known as Air Force Two, that can be pressed into service as an Air Force One if need be for short flights.

Also not particularly unknown are four additional specially outfitted Boeing 747s that have been dubbed the E-4B Nightwatch -- one of which is always on standby -- which doesn't have the same sort of luxurious accommodations as the VC-25s but are instead focused on communications and serve as a sort of airborne command center in emergency situations.

But there is yet another, much smaller aircraft that can sometimes fill in as Air Force One that most people don't know about -- a small fleet of Gulfstream private jets that the Air Force designated as C-20s.

Per Politico, at least one of those typically unmarked C-20s always shadows the more recognizable VC-25s and is prepared to accommodate the president and a small group of staffers at any given moment.

Used for emergencies or secret flights

The C-20s -- which may have been phased out and replaced by now -- generally follow the president wherever they go but take off and land at different airports near the more recognizable VC-25 Air Force One planes.

On a few occasions, such as a secret flight former President Bill Clinton made into Pakistan, the C-20s have quietly been used as Air Force One, but they more often than not are just on standby in case an emergency of some sort makes it impossible or too dangerous for the president to use the standard VC-25 aircraft.

Such occasions are obviously few and far between, and the Air Force typically doesn't even openly acknowledge the existence of the emergency back up aircraft for the president that is nonetheless reportedly always nearby but kept out of sight.