A two alarm fire is an industry term that refers to the amount of units dispatched to the scene of a fire. There is no universal determinate in what constitutes a two-alarm fire. Rather, local jurisdictions set their own qualifications for what constitutes multiple-alarm fires.
A two-alarm fire in New York City, for example, commands 26 response units and a total of 106 firefighters. This is not necessarily the case in other communities, however. Additionally, although "units" generally refers to fire trucks dispatched, they can refer to any type of emergency response vehicle. Boston was the first city to implement the multi-alarm system in 1852, and at the time, gongs were used instead of "alarms." Each strike of the gong advised how many units were needed.