I recently realized that I used to play the foghorn chord wrong (the very first notes in the intro of Down to the Waterline). I played those foghorn sound on only the two lowest strings – this way I explained it in a Youtube tutorial on Down to the Waterline. In fact it is played on three strings. Not a big deal, and I guess many of you were already aware of this, but for me it was again one of those little bits that make such a nice effect, and I simply did not think about it at all before. By the way, the same chord appears in the intro of Radio City Serenade on Mark’s Privateering album.
The chord consists of the notes B, F, and A. With B being the root nore, it is a B7b5 chord (the F is a semitone below the fifth note of a Bm chord – the F# – and thus denoted as b5, while the A is the 7th). It is mainly the b5 that makes the mysterious, misty foghorn association. The interval from the B to the F is a so-called tritone. It was called the ‘Diabolus in Musica’ (devil in music) centuiries ago, and was avoided, almost banned, as it was regarded as evil. Tritone means three whole notes. The tritone divides an octave in two identical intervals, in other words, B – F is a tritone, and so is F – B.