Works that are not protected by copyright are said to be in the Public Domain, and you are free to use them in any way you choose. That means no restrictions on copying and adapting, no need to seek permission, and no uncertainty about your rights as a user. There is also no legal requirement to attribute works in the Public Domain to their creators, although doing so is an important part of maintaining academic integrity.
There are several ways a work might end up in the public domain: (1) when copyright expires, (2) if the copyright owner waves their copy rights and publishes their work to the public domain, or (3) if a work does not qualify for copyright protection (UBC).
Use the Public Domain Flowchart (below) to help determine if a work is in the public domain.
Not sure if a work is in the public domain? Use the flowchart linked below to do a quick analysis.