Hello, fellow nerds! For this edition of Boom! Lawyered, we are going to do a quick and dirty explainer about “stop-and-frisk” policies and the Fourth Amendment.

Since discussions of stop-and-frisk are back in the news because Donald Trump insists that the practice is going to save cities like Chicago, we thought we’d give you the answers to some practical questions: “What is a stop-and-frisk?” “What are cops allowed to do when they stop you?” “When can cops search your car after a traffic stop?” That sort of thing.

The Fourth Amendment, as you likely already know, protects you from unreasonable search and seizure. But like many rights, it is not absolute. Legally, Fourth Amendment concerns must be balanced against concerns about stopping crime and officer safety. As you also likely already know, whatever the police seize can be used as evidence in court, unless the seizure is illegal—in which case, that evidence may be excluded as “fruit of the poisonous tree.”

“Stop-and-frisk” is another term for “investigative detention” or “Terry stop,” from the landmark 1968 U.S. Supreme Court case Terry v. Ohio. Under Terry, a cop may stop you if they have reasonable suspicion that you have committed, are committing, or are about to commit a crime, and may frisk you for weapons if the cop has a reasonable belief that you’re “armed and presently dangerous.” Though “stop-and-frisk” has colloquially come to apply mainly to street encounters, being frisked after your car has stopped is also permissible under Terry.

Terry comes into play when a cop doesn’t have probable cause to arrest you—”probable cause” is the belief that a crime has been committed and that you have committed it—but has “reasonable suspicion” that you’re up to some criminal shenanigans. The Supreme Court has defined “reasonable suspicion” as “articulable facts that would lead a reasonable officer to conclude that criminal activity is afoot.” So, more than an unsupported hunch, but less than “more likely than not” (United States v. Sokolow).

Cops have taken the Supreme Court’s ruling in Terry and run with it. This led to, for example, New York City’s stop-and-frisk program. Contrary to what Trump has repeatedly claimed, the program was deemed not only unconstitutional because of its rampant racial profiling, but also ineffective.

Here are a few things you should know about stop-and-frisk, both in New York City and elsewhere:

• After a cop stops you, they may pat you down for weapons in order to ensure public safety and officer safety. (Terry v. Ohio)
• If a cop frisks you, any items they find that might be a weapon or contain a weapon can be removed from your pockets—or wherever—and inspected. (Terry v. Ohio)
• If a cop stops and frisks you for weapons and feels what is “immediately recognized” as contraband, that contraband may be lawfully seized, even though it’s not a weapon (Minnesota v. Dickerson). For example, if a cop frisks you for weapons and feels an object that he immediately recognizes as a crack vial, that’s a lawful seizure under the Fourth Amendment. If, however, the cop isn’t sure whether it’s a crack vial or a pen cap, but the cop fiddles around in your pocket until they’re convinced that it’s a crack vial, that’s likely not a lawful seizure since the cop didn’t “immediately recognize” the item in your pocket.
• If a cop stops and frisks you, the cop may also lawfully “frisk the lunging area,” meaning to ensure officer safety (by locating potential weapons), they may frisk nearby areas such as a trash can or a purse. (Michigan v. Long)
• If the cop finds a closed container within lunging distance of the person being frisked, the cop can open the container to see if it contains a weapon, if the officer believes the container could contain a weapon and the container is not locked. (Michigan v. Long)

The bottom line is that if the cops don’t have probable cause to arrest you—meaning reasonable grounds to believe that a crime has been committed and that you have committed it—they can still stop you if they think you’re going to commit a crime, and can frisk you for weapons if they think you’re armed and dangerous.

Similar rules apply in traffic stops, or so-called “rolling” or “moving” Terry stops.

Cops may pull over or stop a moving car if they have reasonable suspicion to support the stop (United States v. Cortez). In addition, cops may use a traffic stop as pretext to search for evidence of further criminal activity (Whren v. United States).

In practical terms, this means that cops have practically unlimited authority to harass drivers, particularly those of color. Since there are so many nitpicky traffic laws, it’s not hard for a cop to find some reason to pull you over in the hopes of ferreting out criminal activity.

As with stop-and-frisk, the key question for rolling Terry stops is whether or not cops have reasonable suspicion to believe that you have committed, are committing, or are about to commit a crime. A cop can detain you for a reasonable amount of time in order to conduct a field investigation. A cop cannot detain you for too long without obtaining probable cause to arrest you. According to the Federal Law Enforcement Training Centers (the Department of Homeland Security’s website), many police agencies have adopted an informal 20-minute rule, meaning if the cops can’t find probable cause to arrest you within 20 minutes, they have to let you go.

A traffic violation is enough for a cop to pull you over in the hopes that something they see in your car, or something you say will give them probable cause for a longer detention and more intrusive search.

Here are a few things you should know about rolling Terry stops:

• The duration of the stop must be reasonable. Usually that’s a relatively short period of time, but there are exceptions. If you’ve swallowed a bag of heroin, for example, the cops can hold you for however long it takes for you to “pass” the heroin. That’s what happened in United States v. Hernandez: The Court found that a 16-hour detention was reasonable since the cops had reasonable suspicion to believe that the suspect, who had just arrived from Bogota, Colombia, was a “balloon swallower.”)
• For the duration of the stop, the driver of the car and the passengers in the car are “seized” for purposes of the Fourth Amendment because a reasonable person would believe they are not free to leave. (Brendlin v. California)
• Once you’re stopped, police officers may order you, the driver, out of the car. (Pennsylvania v. Mimms)
• Cops may also order all passengers to get out of the car for officer safety even if the cops don’t have any reason to suspect that any passenger is involved in criminal activity. (Maryland v. Wilson)
• Just as with a stop-and-frisk on the street, cops may also frisk a vehicle passenger if they have reason to believe a passenger is armed and dangerous. (Arizona v. Johnson)
• Cops can order occupants to remain in the car—at least in states in the Third Circuit, the Eighth Circuit, the Ninth Circuit, the Tenth Circuit, and the D.C. Circuit. The Supreme Court has not yet weighed in on whether or not passengers can be ordered to remain in the car, but should that case ever reach the Supreme Court, it’s likely the Court would rule that in the interests of officer safety—preventing a person from exiting a car, leaving the scene, and returning to the scene with a weapon—ordering passengers to remain in the car is not a Fourth Amendment violation.
• Once stopped, cops can visually inspect the interior of your car and search your car for weapons and seize any contraband found during a search for weapons. (Michigan v. Long)

It’s not difficult for cops to come up with reasonable suspicion to justify a stop. Maybe they saw a weapon or felt a weapon when they patted you down. Maybe they smelled marijuana or thought your eyes looked bloodshot. And once the cops justify their reasonable suspicion—they find contraband or you admit to a crime—that can provide the probable cause for a longer detention and a more full search of you or your car.

So there you have it! That’s about all you need to know about stop-and-frisk and the Fourth Amendment.

The most important thing to know is that if the police have reasonable suspicion you might commit a crime, they can ask you questions. If they think you might be armed and dangerous, they can also pat you down for weapons. If the “reasonable suspicion” that you have committed a crime turns into “probable cause” to believe you have, based on information they learn from you or contraband they find on you in the course of patting you down for weapons, they can arrest you.

A frisk for weapons, however, is different from a a full search to find evidence of a crime that a cop thinks you committed. After frisking you for weapons, a cop may ask you to consent to a full search—to use the “pen cap or crack vial” example from earlier, this would allow them to investigate whether the suspicious object in question is contraband. If you consent to a full search and a cop finds something during this full search that gives them probable cause, they can arrest you. This holds true even if you consent to a search because you think you have to. (Yes, cops are allowed to lie and manipulate … to a certain extent. They don’t have to tell you what your rights are; it’s up to you to know what your rights are.)

Overall, you should think carefully before consenting to a search or answering a cop’s questions.

Don’t be like that TV cliché—you know, the wrongfully accused suspect who thinks blabbering all their business to the cops is going to help. It doesn’t help! Don’t you watch movies? Ask for a lawyer and keep your mouth shut.