A more thorough explanation:

Complementary products are goods that are typically used together. For example, pancake syrup and pancake mix are complementary products because they are often used together. Another example is motion-picture projectors and film.

Trademark law may prevent the use of a similar mark on complementary goods because consumers may be confused into thinking that the goods come from a common source. This means that if a company has a trademark for pancake syrup, another company cannot use a similar trademark for pancake mix. The patent-misuse doctrine may provide a defense in an infringement suit if the plaintiff has used its patent rights to gain market control over unpatented complementary goods.

For instance, if a company has a patent for motion-picture projectors, it cannot use its patent rights to gain control over the film market. This is because the patent-misuse doctrine prevents companies from using their patents to gain control over unpatented complementary goods.

Overall, complementary products are important in trademark and patent law because they can affect market control and consumer confusion.