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an opening that provides access to the interior of a structure, assembly, or machine. Under normal conditions of use, a hatch is closed; it is opened only to carry out necessary operations.
On a ship, a hatch is an opening in the deck used for loading operations (cargo hatch), communication with below-deck quarters (companion hatch), or admitting air and light below decks (skylight, or porthole). A hatch in an upper deck is usually watertight, its perimeter framed by coamings.
Hatches (portholes) are also installed in the fuselages of air-craft, in spacecraft, in the floors and ceilings of industrial installations, in boilers, and over manholes for access to water, gas, and sewer mains and telephone systems.