For the majority of human history, the most economical solution was simply to have someone make a tracing of the original plans. In the mid-nineteenth century, the process abruptly became much quicker and easier thanks to famed polymath Sir John Herschel. In 1842, Herschel invented a method to easily copy drawings using potassium ferrocyanide and ammonium iron citrate. In the mid-twentieth century, copying methods such as as diazo prints, and then later xerographic prints, finally supplanted blueprints. Despite the technological changes and the fact that these plans usually aren’t on blue paper anymore, in popular vernacular the term “blueprints” has stuck around anyways.