WHAT?

Which Major and which Minor?

Well it's not obvious.

I've transcribed over 30,000 tracks to get here.

Lets manipulate the modes using a formula.

A common Grunge formula modulates between G Major and G Natural Minor(VI Mode of Bb Major)

or any Major key to a Natural Minor Mode of the same name

lets take the 3 major chords from G Major (G C & D)

and the 3 major chords from G Natural Minor(Bb Major)(Bb Eb & F)

here's the science

You must use G as the first chord. ok.
Then use the other major chords(C D Bb Eb or F)in any order.
The result is instant.You become King of Grunge! ||G / C / | F / Bb / ||
Check out my Instant Nirvana page. It's got a progression generator for this formula.

Ok It's not big enough for a song. sure! Let's add tweak it.
Continue using those major chords, till you fancy a change.
The next chord you use that's not G C D Bb Eb or F, is your new beginning.
Lets say you choose E.
We will take the three major chords from E Major (E A & B) and mix them in any order, after we play E, with the three major chords from E Natural Minor( G Major) (G C & D) we get this: ||E / C / | G / A B ||

These ideas will generate a valid Grunge type song.

recap.................
randomly choose each key
randomly choose the time spent in each key( how many bars)
use the same Major to Natural Minor mode formula based on the first new chord
There are more Songwriting Techniques to explore at my site.