Maybe it's just because they're French (;-)??).  I believe I did hear an
explanation along the lines of the earlier post (which I don't have access
to so can't acknowledge at the moment) referring to riding on horseback
and carrying the horn as being the reason but can't verify.  However, to
confuse the issue, there is a horn that looks like a french horn, sounds
like a french horn, but is muted by the left hand and has trumpet type
valves rather than french horn type stops and the valves are fingered by
the right hand.  That horn is usually used in bands.  I don't know what it
is called - it may just be made that way so that it can be played easier or
may be to get the french horn range of sound in a band type instrument.  We
had that type when I was in school.

[log in to unmask] (Mary Esterheld)