For the 2003 film, see Blueprint (movie).

Modern blueprint of the French galleon La Belle.

A blueprint is a plan or design documenting an architecture.

The name comes from the photographic print, obtained through the cyanotype process, which is composed of white lines on a blue background commonly used in the past for copying architectural plans and engineering drawings.

The blueprint process was developed by the British astronomer Sir John Herschel in 1840. The photosensitive compound, a solution of ammonium ferric citrate and potassium ferricyanide, is coated onto blueprint paper. Areas of the compound exposed to strong light are converted to insoluble blue ferric ferrocyanide, or Prussian blue. The soluble chemicals are washed off with water leaving a light-stable print.

A similar process was used to produce printing proofs for offset printing.

Blueprints have mostly been replaced by Diazo prints or whiteprints, which have blue lines on a white background; thus these drawings are called blue-lines. These are sometimes also known, incorrectly, as blueprints.

Revision control was done in contrasting color on the blue-lines, for example red markup of a blue-line copy by the engineer, then yellow markup on the copy by the draftsman who implemented the changes on the original drawing, then brown markup by the checker, on a check-print (a brown-line). Finally, the architect or engineer, draftsman, checker and supervisor would sign the original drawing, thus making it a legal document.