A song of C Key will usually have chords of: C Dm Em F G Am B However, if you find such song which comes with E chord as well, it doesn't mean that is wrong. It is because of using Harmonic Minor... Here is why: First of all, a song of C Key is having Am as its relative natural minor key. List the scale of Am key: A B C D E F G A <------ This is Natural Minor *** Watch out *** Sharp the seventh note: A B C D E F G# A <------ This is Harmonic Minor Okay no