What is the 'guard' frequency?

Wizardo...

The emergency frequencies are 121.5 and its double, 243 MHz.

A more complete answer. Guard goes a long way back, used first by the
services.

During WWII, one of the four frequencies on the VHF radios of, say, a P-51, was
121.5, the "emergency" frequency. Depending on what stage of the war, some
rescue or relay planes would monitor that frequency and pass along any
emergency messages to England.

There is a lot more history there.

After the war, radios, mostly the UHF type, were built to have a primary
frequency to which they tuned, changing that as they progressed on a mission,
and they were supposed to also enable the "guard" channel such that they would
pick up any emergency mayday and be able to assist.

Airlines also, as they got better radios and/or duplicate radios, were capable
of listing to "guard" in flight and could also pass along emergency messages, a
nice thought when over the ocean.

In the recent past, after the new york event, all flyers were supposed to
monitor the 121.5 in the event there had been another big problem. Since most
use radios which do not have a guard channel which can be monitored at the same
time as a primary frequency, the "suggestion" was not a full time thing for
anyone.

Guard had several meanings, depending on the mission. Some squadrons assigned
a frequency different than the emergency frequency to guard.

Enough?

Neal