I'll add my 2 cents: Suppose I'm transmitting on 500 kHz with a modulated 800 Hz tone. Suppose you have your receiver's BFO set so that a signal on 500 will have a 600 Hz tone. The 800 and 600 Hz notes will create many, many harmonics -- a very wide-band signal on either side of 500. The sound is so distinctive that a radio operator, busy with something else, will immediately get a shot of adrenaline through his system!
If you have a piano handy, try sending Morse with your arm pressing a dozen keys simultaneously. That's what an A-2 transmission would sound like.
ITU regulations stated that all distress transmissions on 500 kHz were to be conducted with A-2 modulated CW.
In US Coast Guard Radioman School, we were told that some 500 kHz Auto Alarm receivers would only respond to A-2 transmissions. (A ship in distress will send an Auto Alarm, a series of 12 four-second dashes, prior to sending the SOS transmission. Those 12 four-second dashes will activate alarm bells on other ships within receiving range.)
Good stuff!
73,
Jeff KH6O and former USCG CW operator at Communications Station Honolulu, NMO, 1977-1980