discoverable for users not already aware of its existence. Around this period, sites with similar features such as CiteSeer, Scirus, and Microsoft Windows Live Academic search were developed. Some of these are now defunct; in 2016, Microsoft launched a new competitor, Microsoft Academic. A major enhancement was rolled out in 2012, with the possibility for individual scholars to create personal "Scholar Citations profiles". A feature introduced in November 2013 allows logged-in users to save search results into the "Google Scholar library", a personal collection which the user can search separately and organize by tags. Via the "metrics" button, it reveals the top journals in a field of interest, and the articles generating these journal's impact can also be accessed. A metrics feature now supports viewing the impact of whole fields of science, as well as academic journals. Features and specifications Google Scholar allows users to search for digital or physical copies of articles, whether online or in libraries. It indexes "full-text journal articles, technical reports, preprints, theses, books, and other documents, including selected Web pages that are deemed to be 'scholarly.'" Because many of Google Scholar's search results link to commercial journal articles, most people will be able to access only an abstract and the citation details of an article, and have to pay a fee to access the entire article. The most relevant results for the searched keywords will be listed first, in order of the author's ranking, the number of references that are linked to it and their relevance to other scholarly literature, and the ranking of the publication that the journal appears in. Groups and access to literature Using its "group of" feature, it shows the available links to journal articles. In the 2005 version, this feature provided a link to both subscription-access versions of an article and to free full-text versions of articles; for most of 2006, it provided links to only the publishers' versions. Since December 2006, it has provided links to both published versions and major open access repositories, including all those posted on individual faculty web pages and other unstructured sources identified by similarity. On the other hand, Google Scholar doesn't allow to filter explicitly between toll access and open access resources, a feature offered Unpaywall and the tools which embed its data, such as Web of Science, Scopus and Unpaywall Journals, used by libraries to calculate the real costs and value of