As Uncle Jesse said on The Dukes of Hazzard - you not only don't know nothing, you don't even suspect nothing!
Back in the day, you had several pieces of equipment connected together with a switch that comprised a radio system.
You had a receiver, its only job was to receive radio transmissions.
You had a transmitter, its only job was to transmit.
You had a switch, that switched the antenna between the receiver and the transmitter.
Look at the picture of most any world war II bomber and you will see two individual pieces of equipment.
You also had another piece of equipment called a calibrator, which was used to tune both halves of the transceiver to the same frequency.
If you didn't have a crystal calibrator, you could use WWV - since it was at the band edge of 40 meters and was a known frequency.
Someone got the bright idea to put a relay inside of a transmitter / receiver, and used a Push To Talk button to key the relay and the TRANSCEIVER WAS BORN!
Now you didn't need to know how to TUNE a radio, and you didn't have to worry that you were transmitting in the wrong band.
If you listen to a old time ham radio operator, they might say - CQ CQ CQ, CQ 40 meters, this is W4JK calling CQ and standing by.
This was because the operator wanted to ensure that he was in fact on 40 meters.
If someone came back and said - hey buddy, you are on 20 meters, he knew he made a mistake.
The old military surplus WW II transmitters could be bought - cheap after the war, and could be converted to single side band, and could be converted so they used 20 meters upper side band, and 40 meters lower side band - by just flipping a switch.
This doubled the amount of bands that they could work with one transceiver and this is the reason why 20 meters is upper side band and 40 meters is lower side band in the band plan.
http://www.radioblvd.com/WWII-PostWar Hamgear.htm
https://www.google.com/search?q=fly...4AOBjoHgAw&ved=0CDMQsAQ&biw=788&bih=484&dpr=1
https://www.google.com/search?q=fly...ic.html%3Fb%3D8%26f%3D9%26t%3D317788;1024;768