Discovering Lydian Mode

I was working on my melody for Chord Progression #7. This chord progression is: C G G D Dsus G G C Dsus D7 (see Songwriting Exercise). I had assumed the chord progression was in the key of C major, because it started with a C major chord. However, what really started throwing me off was the presence of the D major, D sus, and D7 chords. A D major chord has the notes D F# A, and there is no F# in the key of C major. Also, when I finished putting my melody into the sequencer software I use and played it back, it really felt like the ending should resolve to a G chord rather than to a C.

Then it occured to me that maybe the piece is using a different mode. I have only recently learned about modes. I had heard the term Dorian mode before but didn't know what it meant, so I looked it up in a dictionary a few weeks ago. The dictionary told me that a Dorian mode is a scale that uses only the white keys on a piano and starts on the note D. In other words, a mode is a scale that starts on a certain note (in this case, D), but uses the key signature of another scale (in this case C major). There is also a mode called Ionian mode, where the starting note and key signature are the same - this is the case for any major scale (for example, a scale that starts on C and uses the key signature of C).

With this thought in mind, I started wondering if this chord progression was written in another mode. With a little research, I discovered the Lydian mode. The C Lydian scale starts on C but has the key signature of G major - that is, an F#. That explains the D major chords.