Tidal locking
A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This makes one hemisphere constantly face the partner body. Usually, at any given time only the satellite is tidally locked around the larger body. If the two bodies are similar in mass, and their distance apart is small, the tidal force will lock each to the other. This is the case between Pluto and Charon.