blueprint

A blueprint is a reproduction of a technical drawing used primarily in architecture and engineering, traditionally produced as white lines on a blue background. The term also refers to a detailed plan or design.

The original blueprint process, developed in the 1840s by Sir John Herschel, used a cyanotype-based method. A sheet coated with light-sensitive iron salts was placed in contact with an original drawing and exposed to ultraviolet light. The resulting image featured white lines on a distinctly blue field. The technique offered accurate, low-cost reproduction and became the standard for architectural and mechanical drawings for much of the 19th and 20th centuries. Later, other copying processes such as diazo and photocopy-based methods replaced the classic cyanotype blue, but the name "blueprint" persisted.

In practice, blueprints typically included elements such as a title block with project name, author, scale, sheet number, and a revision history; standardized line weights and symbols denoted different features and materials. Drawings often used a fixed scale and were presented as a set on a sheet or as a sheet in a drawing set.

Today, digital tools such as CAD and BIM have largely supplanted traditional blueprints, with most professional drawings produced as vector files and printed as needed. The phrase blueprint endures as a general metaphor for a detailed plan or prototype, even when the physical cyanotype method is no longer used.