Multiple-alarm fire

[3] In terms of understanding the relative severity of an incident, the government of Rochester, New Hampshire has reported in a statement that a "typical fire in a small structure will usually require only one or two alarms".

Subsequent alarms are calls for additional units, usually because the fire has grown and additional resources are needed to combat it, or because the incident is persisting long enough that firefighters on scene need to be relieved.

[5] Requests for units and firefighters from outside jurisdictions do not normally occur in multi-firehouse urban areas until elevated alarms are reached (alarm three and above), but will depend on the location of the incident and the condition of the authority having jurisdiction at the time of the incident.

If the fire is not large enough to require a 2nd alarm, but there is a need for more equipment and manpower, the commanding chief can request additional units to respond "specially called" to the scene.

So, it is not a matter of how many firehouses respond to a fire as popularly believed, but rather how many companies/units and how many firefighters are operating on scene.

The system of classification comes from the old tradition of using pull stations to alert the local departments to a fire in their area.

[3] When initially pulled, a pull box will pulse its identification number multiple times to a receiving reel-to-reel telegraph unit (typically located in the nearest dispatch office).