And, um...wow, this is alot harder than I thought
This needs a complete re-write
The writing about "primary" and "secondary" chords is completely wrong. Perhaps those are legitimate terms in the United States, in which case I'll leave the terms as they are, but the description given is complete nonsense. Primary chords, it says, are so named because "...the notes in each chord can be unjumbled to create the major scale they represent."
So, for C major, the example given, the "primary chords" are C major (C - E - G), F major (F - A - C), and G major (G - B - D). Unjumbling that string (C, E, G, F, A, C, G, B, D), we do indeed get the notes of the major scale (C, D, E, F, G, A, B, C).
Now take the "secondary chords", which are D minor (D - F - A), E minor (E - G - B), A minor (A - C - E) and B diminished (B - D - F). The string of notes created by these chords (D, F, A, E, G, B, A, C, E, B, D, F) can also be unjumbled to create the notes of the major scale.
Also, this version of events, whereby the tonality of the primary chords depends on the key signature, while the secondary chords "carry the opposite tonality of the piece's key signature" is gobbledegook. Given that any key signature can represent a relative major or minor key, how can we say that a key signature "has" a specific tonality? What the page is trying to say is that "in any key, the primary chords share the same tonality as the key (ie - they are major when in a major key, and minor in a minor key), while the secondary chords share the opposite tonality (ie - they are minor in a major key, and vice versa). The only exception to this rule is the chord of VII, which is diminished."
But even this is still wrong, as it only holds true for major keys. In a minor key, the (primary) chord of V is major, while the (secondary) chord of VI is minor. Oh, and the chord of II becomes diminished, while the chord of VII becomes major!
Harmony cannot be taught this way. If you start with a seductively simple generalisation and go no further (as the page currently does), students are left with a severely deficient understanding. If you start with the simple rules and then follow them up with an equal number of exceptions to those rules (which would be the required modification to this page for it to make sense), students become confused and discouraged.
For that reason, I am stripping this page to its bare bones, and I will rewrite it as soon as I can. Eoinc 06:22, 24 April 2008 (EDT)