Composing in a Major Key

Okay, so I'm clear on chords in a minor key; which chords can me derived from which minor scale and how you can mix and match chords from each of these scales to produce different effects. But what about composing in a major key? Can I nick chords from the three (lets limit it to the aeolian, harmonic, and melodic minors for now) scales of the relative minor when still in the major?

For example, if I'm composing in C major, could I "borrow" a G sharp diminished or Caugmented from the relative minor A minor?

Provided you return them when you're finished :D

Yes, you can borrow chords from the Minor key. It's called mode mixture (i.e., mixing the major and minor mode). It was common in the 60s for songs to have I - bVII - IV - I chord sequences, e.g., The Stones 'Sympathy for the Devil'.

Sorry, I just noticed, you said the 'relative minor'. No, mode mixture implies borrowing from the tonic (parallel) minor, not the relative minor. You can do that too, but it's a different thing. If you use G# dim, then you're moving away from the tonic C to the new tonic A, (whether or not you follow it through).

But mode mixture means just changing the mode and using chords from C minor instead of C major. It's mostly just a different flavour. The borrowed chords still relate to the same tonic, unlike if you borrow chords from the relative minor. It's not useful to think of those chords coming from the relative minor. If you play C aug, for example, you've chromatically raised the tonic C major chord's 5th. It's now a very unstable tonic chord with a raised 5th. Don't think of it in terms of a different key, such as Am, unless you're planning on going there.

Understood, props fretsource.