The Multiple Docking Adapter (MDA) is connected to the Structural Transition Section (STS), the forward part of the Airlock Module (AM). The MDA has a loaded weight of 6210 Kg, a diameter of 3m, a length of 5.1 m and a habitable volume of 32.4 m³.

It has two docking ports compatible with the Apollo Command Module (CM). The main port is located on the axis of the MDA at the forward end. This was the only port used during the Skylab missions.

The second port is located on the side of the MDA near the front end. This was provided in case of the event of astronauts being stranded aboard Skylab. A 'Skylab Rescue Mission' would the be flown using a modified CM, with two extra seats, that could be launched with two crew and return with five. It would dock to this emergency port as the main port would be occupied by the primary CM.

The MDA also contained a window for Earth resources observations, a control center for the Apollo Telescope Mount, film vaults, electrical equipment and materials processing experiments.

The Apollo Telescope Mount was supported by trusses on the side of the MDA opposite the emergency docking port. This allowed clear access to both the main and emergency docking ports.