I have a question regarding modes that has been nawing at me. I have a very limited theory vocabulary so things may sound stupid or elementary. I was wondering, say you have a song in C major. I know that the C Ionian contains the same notes as D dorian. Here's the question: Is playing D dorian in a song (key is C major) any different than playing C ionian? or Does the theory of modes come in when the song is in the Key of C major and you play a C Dorian scale over it to add some *spice*?