A modern four-alarm fire refers to the number of units dispatched to the scene of a fire. Most major cities define a unit as a vehicle such as a fire truck, fuel truck, ladder truck, the department SUV or a civilian vehicle. The exact number of dispatched units and firefighters on the scene varies from department to department and rises as the number of alarms increases.
Historically, fire alarms powered by telegraph technology were placed on street corners and conveyed messages between fire departments by code. At that time, two short rings followed by two more rings was the coded request for another fuel wagon. Four rings indicated a request for more units and therefore more fire fighters. The meaning of four-alarm rings varied by location.
According to Slate, "Four rings in a row meant, 'The chief is on the scene.' In general, more rings referred to a more intense fire. A particularly big blaze was denoted by a code of four rings followed by another four and came to be known as a 'four-alarm fire.'" This system evolved from city to city until the invention of more modern emergency notification services, although these old-style boxes are still found in rural areas.