Under the United States’ former first-to-invent system, it can/could prove important to show the date of reduction to practice in order to demonstrate priority. As a result R&D departments, particularly in the pharmaceutical sector, developed a system of inventors’ notebooks.
Traditionally these notebooks were duplicate books, which made a carbon copy of each page, in which researchers noted all their work. At the end of each day the inventor would then find a colleague who would countersign and date the pages of the notebook written that day, sometimes with a statement such as “read and understood by me.” Researchers would keep the retained carbon copies in the notebooks on their office shelves, but the originals would be archived by the company. Modern inventor’s notebooks are often escrowed electronic logs.