Originally Posted by: Guitarbuff1My question is since the 5th of the Key is A (V major), how is it that we have an Am. I thought the minor chords would have been under II and III and sometimes VI. Are there exceptions to the rules around Chords in a Key?
The short answer is that a key signature is a default setting of a song or melody. It is not a list of the only chords you are allowed to use. You can use any & all chords you like the sound of in your music.
The vast majority of music (classical, jazz, blues, pop) uses accidentals; that is, notes not in the key signature. While the key signature provides the main foundation of the music, the accidentals provide variety that adds spice.
To address your specific example, you are in the key of D major. The V chord is A major. But if the composer likes the sound of A minor in a certain point of the song, then he should use it. Accidentals are used in two ways: structurally & ornamentally.
Using an A minor chord in a D major key structurally means that the accidental, the odd note out, the C natural, is used as part of the melody or in some very important functionally way (to voice lead from one chord to the next, for example).
Using an A minor chord in a D major key ornamentally means that the accidental, the C natural, is used as an ornament or "color note", to add some variety to the chords, but not necessarily part of the melody. Or just because the composer or performer likes the sound of it at that point in the music.
This tutorial covers the basics of music theory, including how chords are built from scales to form the harmonic functions of a key signature.
http://www.guitartricks.com/tutorial.php?input=495
These tutorials covers the basics of improvisation, which include using a key signature as the default setting, but how to include accidentals to modulate:
http://www.guitartricks.com/tutorial.php?input=876
http://www.guitartricks.com/tutorial.php?input=483
http://www.guitartricks.com/tutorial.php?input=491
These tutorials cover the basics of blues improvisation, which include lots of mixing major (chord tones) & minor notes (accidentals as ornamental color tones).
http://www.guitartricks.com/tutorial.php?input=918
http://www.guitartricks.com/tutorial.php?input=217
These tutorials are an intro to jazz guitar, which includes using accidentals as a means of modulation.
http://www.guitartricks.com/tutorial.php?input=519
http://www.guitartricks.com/tutorial.php?input=1166
Almost any classical piece has accidentals. Fur Elise is a great example because it contains both ornamental (the D-sharp of the intro) & structural (the G-sharp of the E7 chord leading to the A minor tonic) uses of accidentals.
http://www.guitartricks.com/tutorial.php?input=168
Hope all that helps. Ask more if necessary. Have fun!
