on this site It gives a chord progression in C major by default(C G Am F), and I'm interested in one of the "Alternatives" it offers for the progression: G,D,Em,C
the progression is also quoted in this article, explaining how that progression is fairly common.
my problem is that the D major is composed of D F# A, and F# should not be present because it's not in the key of C, so what am I missing?
Why can a progression involving D major be used as an alternative to a C major chord progression?
BTW the progression sounds horrible if I replace D with Dm ( using D F A ) that should be a chord in the key.