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Interior design is the planning and design of man-made spaces. It is closely related to architecture and is part of environmental design. It is a relatively new field of design, coming into its own during the later 20th century. In those European countries where it is well established as a profession, it is known as interior architecture. In terms of the distribution of responsibilities, the architect usually concerns her- or himself with the overall design of buildings, while the interior designer is concerned with the more intimately-scaled aspects of design, the aesthetic, functional and psychological questions involved and the character or atmosphere of spaces.
It is carried out in two main contexts: residential and non-residential. Many design firms in the non-residential sector have become specialised, for example, concentrating on the design of hotels, stores, industrial parks or shopping centres; college or school projects; or hospitals, clinics and nursing homes. Residential interiors are usually highly personal statements for both the client and the designer, each of whom is involved with all aspects of the design.
Source: https://www.britannica.com/art/interior-design