พิพิธภัณฑ์และโครงการวิกิมีเดียคอมมอนส์Wikimedia Commons

Commons takes only files that are out of copyright or published under a free licence, and its definition of free is strict: a licence must specifically and irrevocably allow anyone to use the material for any purpose. Republication and distribution must be allowed. Commercial use must be allowed. The licence must be perpetual and non-revocable. A file must also be free of copyright restrictions in both the United States and the source country of the work.

What that admits: CC0, public domain, Creative Commons Attribution (CC BY), Creative Commons Attribution-ShareAlike (CC BY-SA), the Free Art License, and free software licences. What it refuses: CC BY-NC, CC BY-ND, CC BY-NC-SA, CC BY-NC-ND, and fair use — in Commons' own words, it "does not accept fair use justifications", and material licensed exclusively under non-commercial-only licences is not accepted either.

Inference — run that rule across the museums in this atlas and the map goes uneven. The Metropolitan Museum of Art put its public-domain images under CC0 in 2017, so a Met amulet can be shown here. The British Museum released 1.9 million images in 2020 under CC BY-NC-SA 4.0, which bars commercial use, so a British Museum amulet cannot. The Pitt Rivers Museum names no Creative Commons licence at all. Wellcome Collection licenses per item, so each picture has to be checked on its own page.

The consequence is worth stating plainly rather than hiding in a caption: a reader looking at this atlas is looking at the collections that chose an open licence, not at the collections that hold the most material.

This atlas accepts CC0, public domain, CC BY, CC BY-SA and the Free Art License, and nothing else. Every image record carries its file name, its licence, its licence URL and its author, and a picture whose licence cannot be read is not published. Candidate files are listed on each record before they are fetched, so the licence is checked at the point of collection rather than at the point of publication.