Open Access Explained: Gold, Green, Hybrid, Bronze and Diamond

Open access means research that anyone can read online without paying. It comes in several forms, usually named after colours, and the labels tell you two useful things: where the free copy lives and what you are allowed to do with it.

The colours

Gold

The paper is published in a journal where every article is free to read, under an open licence. The journal is typically funded by a fee the authors or their funders pay, called an article processing charge. You read gold papers on the publisher's own site.

Diamond

Diamond is gold without the author fee. The journal charges neither readers nor authors and is paid for by universities, societies or grants.

Hybrid

The journal is a subscription journal, but the authors of this particular paper paid to make it free under an open licence. Other articles in the same issue may still be paywalled.

Bronze

The paper is free to read on the publisher's site, but with no open licence. Publishers do this for older issues, for articles of public interest or as a promotion. Because there is no licence, the publisher can put a bronze paper back behind the paywall at any time.

Green

The published version is paywalled, but a free copy sits in a repository: a university archive, a subject archive such as arXiv or PubMed Central, or a preprint server. Green open access is the reason so many paywalled papers can still be read for free, and it is what our finder is mostly looking for.

Closed

There is no free copy that the open-access indexes know about. Our guide to reading research papers for free covers what to try next.

Which version are you reading?

A paper goes through three stages, and a free copy can be any of them.

• Preprint, also called the submitted version. This is the authors' manuscript before peer review. Results and wording may change before publication.
• Accepted manuscript, sometimes called a postprint. This is the text after peer review, as accepted by the journal, but without the journal's typesetting. The content matches the published paper; the layout and page numbers do not.
• Published version, or version of record. This is the final article as it appears in the journal.

For most purposes the accepted manuscript is as good as the published version. If you need to quote page numbers or cite the paper formally, cite the published version using its DOI, which is explained in what a DOI is and where to find it.

Why is there often a delay?

Many publishers let authors put the accepted manuscript in a repository only after an embargo, commonly six to twelve months after publication. This is why a paper with no free copy today may have one later, and why it is worth checking again.

What the licence tells you

Free to read is not the same as free to reuse. Gold, diamond and hybrid papers usually carry a Creative Commons licence, the most common being CC BY, which lets anyone share and adapt the work as long as they credit the authors. Licences with NC forbid commercial use and those with ND forbid changes. Bronze papers and many green copies have no open licence: you may read them, but the usual copyright rules apply to anything further.