A more thorough explanation:

Primary purpose or effect refers to the main reason for or consequence of using a product. It is used as a test to determine whether the sale of a product amounts to contributory negligence. For example, if the primary purpose or effect of a product's sale or use infringes the copyrights of others, its manufacturer could be stopped from selling the product or required to pay a reasonable royalty to the copyright owners.

One example of primary purpose or effect is the landmark copyright case of Sony Corp. of Am. v. Universal City Studios, Inc. In this case, the Supreme Court rejected the test, but four justices said that if the primary purpose or effect of the product's sale or use infringes the copyrights of others, its manufacturer could be enjoined from selling the product or required to pay a reasonable royalty to the copyright owners.

Another example of primary purpose or effect is the sale of a product that is primarily used for illegal activities, such as drug paraphernalia. In this case, the primary purpose or effect of the product's sale is to facilitate illegal activities, and the manufacturer could be held liable for contributing to the illegal activities.

These examples illustrate how primary purpose or effect is used to determine the legality of a product's sale or use. If the primary purpose or effect of a product is illegal or infringes on the rights of others, the manufacturer could be held liable for contributing to the illegal or infringing activities.