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Overall view of the Command Module's forward deck and docking tunnel.

Note that a 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.