I'll try and be brief but it's a pretty complicated topic. If the key of the song is C# major, unless there's a very specific effect you have in mind, you wouldn't use C# minor. Let me explain.
For any given scale, there are seven basic chords which it (normally) will use. They are numbered I to VII in Roman numerals. Think about it - if you go up the notes of the scale there are seven different notes it could be before you're back where you're started.
In all of these chords, the same black notes of the scale apply, otherwise it will sound slightly odd.
Not all of the chords revolving around C# major will be major themselves. For example.
Chord I = C# + E# (looks like F) + G#. Play it. It sounds major right?
NOW
Chord II = D# + F# + A#. It's a minor chord.
If you're song key is major, the
primary triads -- chords I, IV, and V are also major.
Secondary triads (II, III and VI) are minor. The only
tertiary triad (VII) is diminished, but it's
very rarely used.
If any song you have worked out is in a minor key,
almost the opposite applies. Chords I and IV are minor, but V is major.
Most of the time. When early blues came along it used V in the minor but that is a special use when dealing with
modes.
Quote: Wouldnt i have to [use notes out of key] if I wanted to use minors?
For every major scale, 3 out of the 7 chords are minor anyway. Using any other notes outside the scale would be starting to establish a transition, and that's a whole other can of worms.
I hope this has been a bit useful - even though I'm young I'm a bit of a dinosaur when it comes to music. I prefer Bach or Jethro Tull to Coldplay. Since I'm doing (academic) music at university (UK) next year, I'll try my best to help with any other theory questions.
Percy
P.S. speak to any classical musician and s/he'll tell you that C# major is most of the time called Db major. And if it wans't complicated enough, C# major is written out very differently to Db, even though they sound the same.
Told you it was complicated!
