What are the Terry stop and frisk rules?

People in the United States are generally free to move around and explore the country without fear of being interrupted by police. However, the law allows some circumstances where police can temporarily suspend those freedoms in the pursuit of crime prevention or investigation. But where is the line? Obviously, if a cop saw you shove a bag of white powder labeled "crack" into your pants, he'd have a right to stop you and check things out, but what happens when things are a bit more abstract? Well, the answer is "it depends how abstract."

The line that the law has created in order to stop someone is "reasonable suspicion supported by articulable facts," meaning it is enough of a "gut feeling" that the police officer can put it into words and explain to someone. For example, if Officer stops Bob standing outside of a bank because Bob "looked at him funny," this isn't a lawful stop. However, if Officer stops Bob running out of a bank because Bob looked at him nervously while an alarm could be heard in the background and $20 bills fell out of Bob's pockets, suddenly the Officer has a reasonable suspicion that Bob robbed the bank, and can articulate his suspicion in the same way described here.

After an Officer stops someone, they can choose to frisk them if they have a reasonable suspicion that the person they have stopped may have a weapon on them. Note that if a person has been stopped because they are suspected of a crime, then unless they are naked there is likely always a chance they have a weapon on them (like a pocket knife), but whether they are considered dangerous depends on how well you argue it on the exam. Also worth noting, if an officer is performing a stop on a vehicle, they can use the same justification to frisk the vehicle, meaning search the areas within the vehicle that are within immediate reach of the occupants within it. The goal is to reduce the probability that an Officer will be harmed by a surprise attack during an otherwise inquisitive stop.