The two prongs to the reasonableness test stem from a 1967 Supreme Court case known as Katz v United States. Charles Katz made money through gambling activities, which were illegal. The government wiretapped the phone booths from which Mr. Katz was making his bets. The Supreme Court ruled that the Fourth Amendment applied in this case. The Supreme Court ruled the Fourth Amendment protects people and not places. It also applies to oral statements and not just tangible objects. The Supreme Court ruled that Mr. Katz’s right to unreasonable searches and seizures was violated.
In this case, a test was devised to determine if a person’s Fourth Amendment rights were being violated. One test is subjective, while the other test is objective. The subjective test is whether a person believes it is reasonable to have privacy. In this case, was it reasonable for Mr. Katz to believe that his phone calls would be wiretapped, and he, therefore, wouldn’t have privacy? The objective test is whether society believes an expectation of privacy is reasonable in a given situation.
The 1967 Supreme Court case Katz vs. US ruled on peoples' expectations of privacy and what constitutes the 4th Amendment's protections against unreasonable search and seizures. In this case, a man named Katz was convicted of making illegal bets from a public phone booth because he was wiretapped by the FBI. The Supreme Court reversed the decisions of the lower court and the court of appeals that had upheld Katz's conviction.
In writing a concurring opinion, Justice Harlan devised a two-prong test for determining whether an action taken by the government is a reasonable search. First, the subjective prong is the individual's expectation of privacy in the situation. For example, when Katz was in the phone booth, he had the expectation that his phone call would not be listened to by anyone else. The second prong is the objective prong, which is that society would recognize that there is a right to privacy in that situation. In other words, it is widely accepted in society that one can speak by phone without having one's calls wiretapped. This ruling extended the right to privacy to people, not just to places.
I assume that you are asking about the reasonableness test that was established in Katz v. United States. In that case, the Supreme Court created a two-pronged test to determine whether a person had a reasonable expectation of privacy. If such an expectation existed, the person could be said to have been in a constitutionally protected zone.
The first prong of this test is subjective. It asks whether the person was acting as if they expected to have privacy. If, for example, a person leaves something in plain view on the seat of their car, they are not acting as if they expect it to be kept private.
The second prong of the test is objective. It asks whether the expectation of privacy was reasonable. Here, the court must decide whether a reasonable person would have expected that a certain place would be private and undisturbed. For example, courts have ruled that it is not reasonable for a person to put trash out on a curb and expect that its contents will remain private.