There are many types of open access, perhaps because it is such a young movement that it's still developing standards. That said, there are three basic types:
Though green open access generally refers to the post-print of an article, there are three basic version types that can be self archived in repositories:
If authors have signed a Copyright Transfer Agreement (CTA), the publisher's policy will determine which version of an article can be archived in a repository. Most publishers allow some sort of green open access. Authors can check their CTA for this information. SHERPA/RoMEO is a database of publisher copyright policies and self archiving information that authors can use to check which version they may be allowed to archive. Not all journals are in SHERPA/RoMEO and it isn't always current, so authors may also want to check the publisher's website as well.
SHERPA/RoMEO classifies publishers into colors for easy identification:
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