Public domain works are not protected by copyright or other intellectual property interests.
Public domain works do not require a license and you may use the works in any way(s) you need.
Works become public domain in three basic ways:
1. The works are inherently not copyrightable (ideas, facts, numbers, laws of nature, basic chord progressions) or protectable (government documents, unoriginal database compilations).
2. The works are donated for public use by their creators through a Creative Commons license (CC0).
3. Copyright time frames have expired.
The following tool identifies copyright expiration based on publication dates.
2012 Michael Brewer and ALA Office of Information Technology Policy
Appropedia's Public Domain Search (PDS) is a custom search engine which searches sites that contain mostly or completely public domain content. It is always best practice to verify the results before assuming the data is public domain.