"Unpaywall consists of a database that includes a list of almost 20 million freely available scholarly articles. Most researchers access it using a browser plug-in that was released in 2017. The service works by searching for a queried paper’s unique digital tag — a string of numbers and letters known as its DOI, or digital object identifier — against those of articles gathered from 50,000 journals and repositories.

"In June 2017, Unpaywall was integrated into a popular science search engine called Web of Science, which is operated by Clarivate Analytics. Dimensions, a service run by Digital Science that launched this year, used Unpaywall from the get-go. These companies, and now Elsevier, pay a subscription fee for a feed of Unpaywall’s database that is updated weekly."