Terry v Ohio

Terry v Ohio, 392 US 1
Police officer who observed conduct by defendant and another consistent with hypothesis that they were contemplating daylight robbery, and who approached, identified himself as officer, and asked their names, acted reasonably, when nothing appeared to dispel his reasonable belief of their intent, in seizing defendant in order to search him for weapons, and did not exceed reasonable scope of search in patting down outer clothing of defendants without placing his hands in their pockets or under outer surface of garments until he had felt weapons, and then merely reached for and removed guns.

Rule: Where police has reasonable suspicion of criminal activity and reasonable concern that suspect has a concealed weapon, officer can pat down for a "frisk" search for safety of officers and others.
But this has devolved to "pat down at will"