"Why do some guitar chord progressions have a major second chord when it should be minor? What is a secondary dominant?"
If you know anything about guitar music theory you know that building chords from the major scale produces the following chord sequence:
1. I major
2. ii minor
3. iii minor
4. IV major
5. V major
6. vi minor
7. vii minor (flat five)
But songs with major two chords are fairly common on guitar. The songs listed below are just a few examples:
"That'll Be The Day" Buddy Holly - Key of A but includes a B major 2 chord.
"Hey Good Looking" Hank Williams - Key of C but includes a D major 2 chord.
"Patience" Guns and Roses - Key of G (gtr. tuned down 1/2 step to Eb) but includes an A major 2 chord.
"Out of My Head" Fastball - Key of E but includes an F# major 2 chord.
A major 2 chord is actually a key change and stems from the music theory behind a functioning dominant seven chord. A dominant seven chord (which can be referred to as simply 7) is a major chord with a flat seven interval. This occurs naturally on the fifth scale degree in a major scale. The dominant seven 5 chord has a bit of tension that leads to and resolves on 1 (the 'tonic' in a major scale). For example, in the key of G a D7 chord leads to and resolves on G. D is said to be the 'dominant' of G major. In fact, D can lead to G whether or not the guitar chord actually has the seventh interval in it.
"Hey Good Looking" by Hank Williams is in the key of C and normally has a D minor chord, but the song uses a D major instead which creates a strong pull to G. When playing this song on guitar you're in the key of C but you're borrowing the dominant from the key of G in order to produce the dominant pull to and resolution on G. Get it? This is said to be a secondary dominant chord and is a composition technique that can be used in any key. So the song examples I used can be explained like this:
"That'll Be The Day" B major is the dominant of and leads to E.
"Hey Good Looking" D major is the dominant of and leads to G.
"Patience" A major is the dominant of and leads to D.
"Out of My Head" F# major is the dominant of and leads to B.
You can create secondary dominant movement for any chord in a key. Just remember that it's a type of key change so the scale you play over it with should follow. This is important when learning music theory for guitar.
