[kroger]

Nice post, I'm looking forward for the next one. Meanwhile I'll give my 2 cents.

The main problem in analyzing tonal music is that we mainly listen to relations between chords. For instance, in the following progression in C major, A major functions as a dominant of D (D is the dominant of G and G is the dominant of C):

    C A D G C. 

OTOH, in the following progression the same A is the subdominant of E:

    E A B E. 

This means that if a song modulates or there's a tonicization [1] the same chord will have different tonal functions and we'll listen to it differently. Just counting a chord in a song may not be enough if they have different functions.

The number of repetitions also matters. Tonally, the progressions C | C | C | G | G and C | G | C | G are the same as C | G. Is he eliminating repetitions in the analysis?

About using A major in C; you can use it as a dominant of D (see my 1st example) or as a chromatic mediant [2] in C major. Of course, in modern music you can use anything you want, but these two are the most common uses.

And, naturally, the types of chords used will vary according to the music style.

[1] http://en.wikipedia.org/wiki/Tonicization [2] http://en.wikipedia.org/wiki/Chromatic_mediant

[jbert]

Doesn't he allow for this by normalising all songs to the key of C before doing the analysis?

[colomon]

The grandparent is talking about modulation within a song. Unless they normalized each section of each song to C without saying so, modulation is still an issue. For instance (though I greatly doubt it's on their list) Jethro Tull's "The Whistler" has one key for the verse, a different one for the chorus, and yet another for the whistle solo...

[HTryan]

Right, all of the songs were normalized to C, which exactly the point. At Hooktheory.com you can examine all of the songs in relative notation (i.e. I IV V in place of C F G) which makes it easier to compare chord progressions without having to bother with their respective keys.