In article ,
KC4UAI wrote:
I understand that there is a convention born of convenience when SSB
was just getting started. Apparently it was easier to build multi-
band rigs for SSB that did upper and lower side band on various bands
when this all got started but most of us don't build our rigs today.

The story I heard is that this started because Collins radios had an IF
of 9MHz and only needed one set of (expensive) sideband filters to have
LSB below 9 and USB above. Mixing to get the final output: F1+F2 gives
same sideband you start with, F1-F2 inverts.

That's what I was told.