They discovered something else of interest: yellow dots in a roughly rectangular pattern repeated throughout the page. They were barely visible to the naked eye, but formed a coded design.

Patternalia: An Unconventional History of Polka Dots, Stripes, Plaid, Camouflage & Other Graphic Patterns by Jude Stewart is out from Bloomsbury Publishing on October 13.courtesy Bloomsbury ...

A subtle background pattern generated by color laser printers helps authorities track counterfeit money or other documents back to the printer--and to the person who pressed the Print button.

I usually guess from the style of the dress.Before 1880 or so, patterns were made of tissue and folded with a sketch and instructions.Then, a pattern envelope was used.