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Overall view of the interior of the docking tunnel and docking ring (at the end of the tunnel).

Picture 1 of 2.

Inside the docking tunnel, at approximately 11:00 and 1:00, are two latch locking interfaces to which the latches on the forward tunnel hatch attach. Between them and farther aft is a handhold to assist the astronaut in maneuvering between the Command and Lunar Modules

The docking ring was added to Apollo 13 for display purposes.

During a mission, the docking ring was used to secure the Command and Lunar Modules. The docking ring, along with the docking probe, was jettisoned with the Lunar Module. On most missions, this took place in lunar orbit (as it is no longer needed and was only dead weight to accelerate during Trans-Earth Injection), but during Apollo 13 (where the LM served as a lifeboat), this took place near Earth, shortly before reeentry.

Additional information about the docking ring and the overall docking system can be found in the Apollo Experience Report: The Docking System.