What is fair use, really?

The four factors, and why none of them are a checklist

US courts weigh purpose and character of use, favouring transformative, educational, critical or nonprofit uses over uses that simply substitute for the original. They weigh the nature of the copyrighted work, with factual works getting less protection than highly creative ones. They weigh the amount and substantiality used, both quantity and whether the 'heart' of the work was taken. Finally they weigh the effect on the market, asking whether the use harms the copyright owner's ability to sell or licence the original.

No single factor decides a case, and courts explicitly reject bright-line rules like 'under 30 seconds is always fine' or 'crediting the source makes it fair'. Two uses that look similar on the surface can come out differently depending on context, commercial intent and market impact.

What fair use is not

Fair use is not a licence you can claim in advance, and it is not a defence that guarantees you win if sued, it is an argument you make after the fact if a copyright owner challenges your use. Adding a disclaimer such as 'no copyright infringement intended' has no legal effect whatsoever. Fair use also does not apply to trademark disputes, only copyright, and it is a US doctrine specifically, not a global standard.

The EU equivalent is narrower and different

The EU and UK do not have fair use. Instead they have fair dealing, a narrower set of specific, enumerated exceptions such as criticism and review, quotation, parody, news reporting and research, each with its own defined conditions. Fair dealing does not weigh open-ended factors the way US fair use does; a use either fits one of the listed exceptions or it does not. This means a use that might survive a US fair use challenge could still infringe under EU or UK law if it falls outside the specific listed exceptions.

What actually protects you if you are unsure

If your use does not clearly fall within a strong transformative purpose, such as genuine parody, criticism or commentary that adds new meaning, get a licence instead of relying on fair use or fair dealing. A short synchronization or reproduction licence is cheap compared to litigation risk, and it removes the uncertainty entirely rather than leaving you dependent on how a court might weigh four open factors after the fact.