A blueprint is a reproduction of a technical drawing or engineering drawing using a contact print process on light-sensitive sheets introduced by Sir John Herschel in 1842. The traditional white-on-blue appearance of blueprints is a result of the cyanotype process, which allowed rapid and accurate production of an unlimited number of copies of an original reference. It was widely used for over a century for the reproduction of specification drawings used in construction and industry. Blueprints were characterized by white lines on a blue background, a negative of the original. Color or shades of grey could not be reproduced. The process is obsolete, initially superseded by the diazo-based whiteprint process, and later by large-format xerographic photocopiers. It has since almost entirely been superseded by digital computer-aided construction drawings. The term blueprint continues to be used informally to refer to any floor plan (and by analogy, any type of plan). Practising engineers, architects, and drafters often call them drawings, prints, or plans.

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