Plain feel doctrine permits a law enforcement officer to seize an object while conducting a legal pat-down search if its nature is immediately apparent during a touching permitted by the Fourth Amendment. The plain-view doctrine is applied by analogy to cases where an officer discovers contraband through the sense of touch during an otherwise lawful search.
The standard was set in the case Minn. v. Dickerson, 508 U.S. 366 (U.S. 1993) where the court held that "when an officer is justified in believing that an individual whose suspicious behavior he is investigating at close range is armed and presently dangerous to the officer or to others, the officer may conduct a patdown search to determine whether the person is in fact carrying a weapon. The purpose of this limited search is not to discover evidence of crime, but to allow the officer to pursue his investigation without fear of violence. Rather, a protective search -- permitted without a warrant and on the basis of reasonable suspicion less than probable cause -- must be strictly limited to that which is necessary for the discovery of weapons which might be used to harm the officer or others nearby. If the protective search goes beyond what is necessary to determine if the suspect is armed, it is no longer valid under Terry and its fruits will be suppressed."
Plain feel doctrine is also known as plain touch doctrine.