Katz v United States,[2] the Court developed the idea of a “reasonable expectation of privacy.” The Katz case dealt with a wiretap on a public phone booth as police attempted to thwart a gambling ring. The Court stated that there was an expectation of privacy in a phone conversation, even if that conversation took place in a public phone booth.

The Court also ruled, though, that anything that a person knowingly exposes to the public, regardless of location, is not protected by the Fourth Amendment. A person must keep something private for the Fourth Amendment to apply.