It’s not so much a bell, really, as an electronic horn, short and shrill. When it goes off, firefighters freeze and listen for the sound that comes next. Usually, only words follow. “Engine 1,” the dispatcher might say—or “Engine 8” or “Ladder 5,” but only one truck—before reciting an address and a task. One tone signals a medical run or some minor emergency, like going out to stabilize a car-crash victim or a coronary case until an ambulance arrives, breaking a toddler out of a locked-up Taurus, or squirting water on a flaming car. Milk runs.

Sometimes, maybe every fifth time, a second tone will follow the first. Two tones is more serious, perhaps a fire alarm ringing somewhere, probably triggered by nothing more than a stray wisp of cigarette smoke or a burp of electrical current jiggling a circuit. Dispatch sends two engines and one ladder truck for those, picking whichever units are available and close.

Even rarer is three tones. Three tones means a reported structure fire, a house or a condo or a strip mall already blowing smoke into the sky. Three tones means blazing orange heat, black smoke, and poison gas; sirens and lights and steam and great torrents of water; men ripping into walls with axes and long metal spears, smashing windows and cutting shingles from roofs, teetering on ladders a hundred feet long. It doesn’t always turn out that way, but three tones, at least, offers the chance of action. Firefighters love a triple.