Originally Posted by: hdoranIn the following song tutorial, Geoff indicates that the song, Are you gonna be my girl, is in the key of A minor.

http://www.guitartricks.com/lesson.php?input=15283&s_id=1053

A minor is the relative minor of C major and so I believe (other than the root note), all notes and chords of C major are in A minor.

Now, the song has as its first chord A major, which is not a chord in the key of C.

The song also includes, in the chorus, a D major barre chord on the 10th position. Again, D major is not a chord in the key of C (or A minor).

Have I misunderstood relative minors and their relationship to the relative major or is there another reason why these chords are in the key of A minor?

Sometimes musicians use chords out of the key. These would be called secondary dominant chords. I am not sure about the particular song etc but it often helps to look at what comes after those chords to see what is going on.

After an A would maybe be an Em and after that D a G would sound good.