It's hard to credit now, but at one time the dominant seventh chord was a big no-no.

The third and the seventh of a dominant seventh chord (assuming a G7, B and F respectively) form a "tritone" (an interval of three whole steps, i.e., "three tones"). This is a dissonant interval and it's an awkward interval to sing, so it was never used in church music (things like Gregorian chant). In fact it was referred to as "the devil in music" and it was avoided like the plague. But—Claudio Monteverdi hauled off and used it in a madrigal arrangement of "Sumer is icumen in" right at the very end, just before it resolved to the tonic. He wanted that "drop the other shoe" effect and that was an effective way to get it. So, mark it down in your musical trivia book: Monteverdi was the first composer to use a dominant seventh chord. This was around 1635 or so, and it was really radical and avant garde at the time. Some people were startled and outraged. In fact, one critic said, "The human ear will never grow to tolerate such dissonance!" Nevertheless, more and more composers used it, the human ear did grow to tolerate such dissonance, and now it's commonplace.

The tritone is the famous "flatted fifth" in jazz (it's either a diminished fifth or and augmented fourth, depending on how you spell the interval), and there it sits, in every dominant seventh chord. By itself, a tritone is ambiguous. It can shrink to become a major third, or it can expand to become a minor sixth (a diminished seventh chord is made up of two overlapping tritones, and that's really ambiguous!). But combined with the other notes of the dominant seventh, it points strongly to the tonic of the key its in, so it's a strong closing cadence or a good way to modulate into another key.

When letter-name chord notation started I'm not sure, but I pretty sure it's fairly recent (maybe sometime in the 1800s or even more recent). Other than written music, classical musicians usually use a Roman numeral system for harmonic analysis. For example:—

I, ii, iii, IV, V, vi, viio, and I would be C, Dm, Em, F, G, Am, Bdim, and C in the key of C —or G, Am, Bm, C, D, Em, F#dim, and G in the key of G. Added notes and inversions would be indicated by small superscript and subscript numbers to the right of the Roman numeral. The advantage of the system is that it works for any key.

By the way, Murray, in your post above, 27-Jun-02 - 08:02 PM, third paragraph—this is the most reasonable sounding explanation for "dominant" that I've heard so far.

Don Firth