The word blueprint comes from a 19th-century printing process. Drawings were made on translucent paper, then exposed to light with chemically treated paper underneath. The result was a white line on a blue background — a blueprint. The technology is gone, but the name stuck.

Today, a blueprint is any detailed technical drawing used to guide construction. It shows plans, elevations, sections, and details, often at multiple scales. Dimensions, notes, and symbols tell the builder exactly what to build and where.

Blueprints are legal documents. They are part of the contract between owner and contractor. Changes must be recorded and approved. As-built drawings reflect what was actually constructed, which may differ from the original design.

Most drawings are now digital, plotted on large-format printers or viewed on tablets. But the old term persists, a reminder of the days when every construction site smelled faintly of ammonia and blue paper.