I'm working through "Guide to the Practical Study of Harmony" by Tchaikovsky and I had a question about this section on triads (he's working with a C Major scale):
The whole mass of major and minor triads may be grouped into three sets of two triads each: One, the tonic group viz: the triads on the 1st and the 6th degrees. Two, the dominant group viz: the triads on the 5th and the 3rd degrees. Three, the subdominant group viz: the triads on the 4th and 2nd degrees.
Now, I get why the 1st and 6th triad are in the tonic group. I also get why the 5th triad is in the dominant group and why the 4th triad is in the subdominant group. But, I'm having a hard time understanding why the 3rd triad is dominant and the 2nd triad is subdominant.
I'm not sure how I'd categorize them, and I suppose they have to placed somewhere, but I'm not sure why they're being placed here.
I Googled around some and found some conflicting information on Open Music Theory which has muddied the water further:
If you are already comfortable with Roman numerals, you can generally think of I, III, and VI as tonic, II and IV as subdominant, and V and VII as dominant. (Though, as you will see below, there is more to it than that.)
I suppose I'm just looking for some clarity here. It's important that I understand why Tchaikovsky is placing the 3rd and 2nd triad where he is as I keep plowing through the book. Thanks for the help!