Jim, did you get the answer to your original question? It has to do with the fact that the initial SSB transmitters used filters around 5 MHz and a VFO around 9 MHz. Adding the two put you on 20 meters. Subtracting the two put you on 75 meters, but with the sideband inverted. Dave -----Original Message----- From: Haynie, Jim (President) Sent: Thursday, August 05, 2004 4:57 PM To: arrl-odv Subject: [ARRL-ODV:11043] Re: Interesting Info. http://en.wikipedia.org/wiki/Amateur_radio I found this while trying to determine why hams use LSB on 75 and 40 and USB on 20,15 and 10. Jim