So glad you asked! It's pretty much on a case-by-case basis, and there is no easy guidelines to tell whether or not infringement has taken place.
Fair use means that you get a legal OK to use copyright works under very specific conditions. The U.S. Copyright Act uses four key criteria to determine if a use is fair, or if it's copyright infringement:
- Is the use for commercial or non-profit/educational purposes?: You're more likely going to get a "fair use" decision when the use is benefitting non-profit or educational purposes (though, of course, not always!)
- How does the new work relate to the copyrighted work?: A journalistic or newsy work based on the copyrighted work is more likely to get a "fair use" decision than a creative work based on the original.
- How much of the copyrighted work did you use?: Using a little vs. a lot of the original work matters when determining fair use.
- How does the use of the copyrighted work affect its marketability?: Does the new work mean the original won't sell?
To get a little deeper into the subject, head to the U.S. Copyright Fair Use Index, a government website that discusses various court decisions related to fair use principles. The website is also the source of all the information above.
