Original Wording

• The Fourth Amendment of the U.S. Constitution deals with two subjects: searches and warrants. The first part of the amendment is concerned with citizens' protection from "unreasonable searches and seizures." The second half details the requirements to be met for a warrant to be issued. It states that "no warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized." This is detailing the condition to be met (probable cause), the required vetting (oath) and the specifics of the warrant (what exactly is being sought).
  
  

Federal Standard

• Originally the standards were only applied to federal agents and agencies. It was not enforced at the lower levels of law enforcement. FBI agents were required to provide probable cause when pursuing warrants, but state and local police departments were not.
  
  

Local Police

• In 1961, a Supreme Court case, Mapp vs. Ohio, made local and state police answerable to the same standards as federal agents and also liable for the same penalties in the case of failure to follow those standards. This was a sea change in environment and procedure for local agencies. These decisions were made according to local standards and could vary from one department to another. This created a consistent national standard to be followed.
  
  

Plain View

• Related to probable cause is the doctrine of plain view. This states that anything viewed by an officer, from a place he is legally entitled to be to have that view, can be seized without a warrant. This applies to situations like seeing through an open door or window. This can also be referenced in situations like traffic stops where the interior of the car is in sight. Probable cause still applies as the officer must reasonably believe the viewed items to be contraband before seizing them.
  
  

Open Field

• Similar to plain view, the open fields doctrine refers to public spaces such as vacant lots, pastures and bodies of water. This states that law enforcement agencies may search areas of this type without the restrictions of probable cause.