Attributing works in the public domain

Works might be in the public domain typically for any of the following reasons:1

Since depending on the jurisdiction in which you are based the situation might vary, you might need to attribute the work even when the work is in the public domain. Additionally, a good idea is to follow what is suggested in the Public Domain Usage Guidelines prepared by Europeana, the aggregator of cultural heritage in Europe. 

In certain cases, authors might choose to waive all their rights, and make it optional to attribute them or not. That’s the case of the authors that choose the CC0 tool (“Creative Commons Zero”), a public domain dedication that puts the work in the public domain once the author has decided to apply the tool. In this case, you can decide whether you attribute the work or not.

1 Additionally in the US works might be in the public domain due to failure of registration prior to the granting of automatic protection, but that situation is very specific and won’t be explored here.

2 This is the case not only for authors using CC0 but also for certain types of work, such as works made by US officials in the context of their employment. However, that rule only applies under US jurisdiction, so make your own risk assessment to whether use or not any of those resources.