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The Commons was started as a project between the Library of Congress and Flickr to increase access to archival photographs. Since it has spread to other historical or heritage institutions and has created a large pool of public domain, historical photographs.
Creative Commons Licenses allow creators to set different attribution, commercial, derivative and sharing restrictions to their work, which often makes reusing or sharing content easier than traditional copyright, while maintaining authorship. In an advanced Flickr search you can limit your results to display only CC licensed works.