Having a hard time finding any info on this subject. Seems to make a good trivia question too, for that reason. I know nothing technically about music, so that adds to the confusion too.
Lots of American cars use a HIGH and a LOW note 2-horn setup, which combine for a nice harmony that also amplifies the sound over either standing alone. For my truck, I went one better & installed A, C, D, & F note horns years back, so it would have a sound that stands out when I need to use it. All 4 notes together sound like a Cadillac (I believe they use the same 4 notes, but they don't tell you that). If you combine different notes that do not "match", you can get VERY irritating & unpleasant sounds. The A, C, D, F combo is a perfect harmony. I can video it, on request.
Now I know one of the Hi/Lo combo is an F note. What is the other note?
I wish horn manufacturers would list this in their specifications so we know what the hell we're buying. I'm picky with my automotive stuff!