Recent estimates suggest paywalls on the web limit access to three-quarters of scholarly literature (Piwowar et al., 2018; Khabsa et al., 2014; Bosman and Kramer, 2018).
The open access movement strives to remedy this situation (Tennant et al., 2016).
In this study, we use the term “toll access” (also known as “closed access”) to refer to paywalled literature (Suber, 2017).
On the other hand, we refer to literature that is free to read as “open access”.
The website Sci-Hub enables users to download PDF versions of scholarly articles, including many articles that are paywalled at their journal’s site.
Sci-Hub has grown rapidly since its creation in 2011, but the extent of its coverage has been unclear.
Sci-Hub is currently served at domains including https://sci-hub.hk, https://sci-hub.la, https://sci-hub.mn, https://sci-hub.name, https://sci-hub.tv, and https://sci-hub.tw, as well as at scihub22266oqcxt.onion — a Tor Hidden Service (Dingledine et al., 2004).
Elbakyan described the project’s technical scope in July 2017 (Elbakyan, 2017): “Sci-Hub technically is by itself a repository, or a library if you like, and not a search engine for some other repository.
Furthermore, we discuss two variants of open access: “libre” and “gratis” (Suber, 2017; Suber, 2008).
Libre open access refers to literature that is openly licensed to allow reuse.