What is the most important guideline for copying materials under fair use?

The guideline that emphasizes copying only what is necessary under fair use is foundational to the concept of fair use itself. Fair use allows for limited use of copyrighted materials without permission from the copyright holder, and one of the key factors in determining whether a particular use qualifies as fair is the amount and substantiality of the portion used in relation to the work as a whole.

By focusing on copying only what is necessary, a person is more likely to stay within the boundaries of fair use. This principle encourages careful consideration of how much material is taken and for what purpose. For instance, if a teacher needs to provide students with excerpts for educational purposes, then selecting only the portions that are essential to meet educational goals aligns with fair use guidelines. This approach helps in respecting the rights of the original creators while still facilitating access to educational resources.

In contrast, copying everything available, large excerpts, or sharing copies widely could potentially infringe on copyright, fall outside the fair use provisions, and adversely affect the market value of the original work.