Fair use factors

The factors come from section 107 of the US Copyright Act: the purpose and character of the use, including whether it is commercial and whether it transforms the original; the nature of the work copied; how much was taken and how central that part was; and the effect on the market for the original. Courts weigh all four together and no single one decides a case. Two misunderstandings are common. Fair use is a defence you argue after the fact, so it can never be held in advance like a permission. It is also a US doctrine. The UK equivalent is fair dealing, which covers only a fixed list of purposes such as criticism, review, quotation, news reporting and parody.