As Officer Joseph Cooney’s ears register the gentle rapping of metal on wood, he begins patting himself down in search of a concealed weapon: a small disc, about the size of a silver dollar, embossed with the Philly skyline trained in crosshairs.

“Someone will take a coin out and start tapping the bar, then another will tap,” says Cooney, a 20-year veteran of the Philadelphia Police Department. “Pretty soon, you’ll hear someone go, ‘Coin check!’ That’s when you’ll see someone slowly backing away, like, ‘I gotta go to the bathroom.’”

The colleague performing that evasive maneuver most likely forgot to bring their “challenge coin,” issued to current and former members of the PPD’s SWAT unit. And there’s a penalty for being caught empty-handed: You must buy the entire group its next round of drinks.

A tangible token of fellowship, the challenge coin was born of the Armed Forces, and using them to ambush drinking buddies in this manner is a time-honored practice. They’ve been adopted in law enforcement, emergency services and the trades. More recently, however, the tradition has been co-opted by liquor families like Distillerie Fratelli Branca as a way to foster IRL camaraderie among its biggest fans.

There are multiple challenge coin origin stories where the military’s concerned. The most commonly told tale involves an American ace shot down over Europe in World War I, only to be detained by French allies who erroneously believed he was a German sympathizer. He was able to prove his identity and save his skin by producing a solid bronze coin bearing the marks of his flying squadron, a gift from his commanding officer. One of his Gallic captors recognized it, called off the firing squad and gave the pilot a bottle of wine.

A later legend involves talk of the challenge coin as it pertains to pfennig (penny) checks. During WWII, American GIs occupying Germany after the Nazi surrender would mess with each other at local taverns, seeing who among them wasn’t carrying a pfennig, a now-defunct form of small-denomination German currency, in their pockets. These were worth so little at the time compared to the U.S. dollar that anyone holding onto one was likely broke; if you didn’t have one, the thinking was you were rich enough to refill everyone’s steins.

Inspired by the military, the [Fernet Branca] coins are manufactured in scant 100-count batches, and have become a commodity among industry cognoscenti.

As you might suspect, there’s not much hard evidence backing any of this up. But that takes nothing away from challenge coins and their symbolic status within the military community.

One ex-Special Forces Project DELTA vet who declined to be named in this piece—“We’re quiet professionals . . . we’re not the SEALs”—believes the challenge coin has its roots in the proto-CIA Office of Strategic Services, as well as in the 10th Special Forces Group, an elite squad of operators that’s been around for more than 60 years. Today, every branch of the military uses them—and the bar isn’t the only arena in which they’re brandished.

“I’ve seen the whole system used as a way to figure out who has to do some particularly shit task,” says Eamonn Connor, a former Army sergeant who served in Iraq as an Explosive Ordnance Disposal tech (think, The Hurt Locker). An alternative to drawing straws, coin checks could decide who among Connor’s fellow soldiers would get stuck with a joyless chore, like running maintenance checks on crappy old Humvees.

It could be way worse. A Navy lieutenant who’s stationed in Southern California told me a story about a deployment during which his vessel was terrorized by a “phantom shitter”—in military parlance, a scatological prankster who defecates in unexpected places around the ship. Coin checks often determined who would be on cleanup. “Really, really bad stuff. That’s a deep, dark hole,” says the officer, whose comparatively cheery coin is emblazoned with the key values of the Navy—honor, courage, commitment—beneath a bald eagle perched on an anchor.

The eventual migration of challenge coins from the military to the police makes sense, since so many cops come from that world. Scott Turdo, a New Jersey corrections lieutenant who produces custom challenge coins as a side business, estimates that 90 percent of his clientele is law enforcement. But he also fills requests from firefighters, EMTs, private business owners, fraternities and sports teams. And I’ve talked to tradesmen who have their own variations on the challenge coin, including an electrician whose crew would carry mahogany “beans” that functioned in an identical manner in a social drinking setting.

The practice has crept over to the service side of the bar, as well. A few years ago, the Milanese makers of Fernet Branca began producing specialty coins, asking reps to hand them out to influential bartenders in American markets where their amaro does well, like San Francisco, Seattle, New York and New Orleans. (They’ve since taken the program global.) Inspired by the military, the coins are manufactured in scant 100-count batches and have become a commodity among industry cognoscenti. Playing off the reputation of Fernet as the “bartender’s handshake,” the brand encourages coin holders to challenge one another for shots, especially when they’re visiting unfamiliar cities.

This appropriation of tradition has been met with a tepid response from some in uniform. “Honestly, as someone in the military, I think that’s kind of lame,” says the Navy lieutenant. “It’s about more than a game.” Hoodwinking friends for free drinks is clearly the most fun function of these coins, but they represent something loftier than that, at least according to those who make them part of their everyday carry.

“It’s a rite of passage. What happens on our last day of [SWAT] training here: You get Tasered, pepper-sprayed and tear-gassed,” says Cooney, the Philly cop. “And after all that’s said and done, they turn around and give you your coin.”