Bill Putney wrote:
>
> Ted Mittelstaedt wrote:
>
> > However the problem is that if you use the wrong notes, like A & D, you
> > create I think it's called a dissonant chord, which doesen't sound right.
> > Basically the 2 notes aren't harmonics of each other,...
>
> Is that what's intentionally used on fire trucks?

I wouldn't think that a major chord would be jarring enough. It would
sound kind of silly on a car horn. I think the standard car horn has a
minor third in it.