Why are Pluto and Charon sometimes called a double system?

Pluto and its largest moon, Charon, are often referred to as a double system because of their significant size and mass relative to one another.12

While most moons are much smaller than their parent planets, Charon’s diameter is more than half that of Pluto, and its mass is more than one-tenth of Pluto’s. Because of this, the two bodies do not simply revolve around Pluto; rather, they both revolve around a common center of mass located between them. This dynamic, combined with their close physical relationship, leads many scientists to characterize the pair as a double system.132