The alarm system for fires is not precise or uniform, though more alarms correspond to a more severe fire. An increase in the number of alarms corresponds to an increase in the requested number of units and fire fighters. Each fire department determines how its alarm system is implemented.
Most cities dispatch no more than five units to a one-alarm call. A unit is a single fire response vehicle, such as a fire engine, ladder truck or civilian vehicle. In New York City, 12 units are sent to a one-alarm call (referred to as an "all-hands" call), 25 units and 106 fire fighters are sent to a two-alarm call and 33 units and 138 fire fighters are sent to a three-alarm call.