Secondary Dominants
A dominant chord wants to resolve to 4 of the chord. This goes for any dominant chord in any key. Now, if that resolution chord happens to be a 1, 2, 3, 4, 5, 6, or 7 then that is great. But, make sure you understand exactly where a dominant chord wants to resolve to.
But what happens if we make one of the Minor chords (ii, iii, iv) a MAJOR chord? It becomes what is called a Secondary Dominant!
Let's look at some examples:
C / C-Eb-G (C minor chord)
C / C-E-G (C major chord, Eb becomes E, which is the leading tone in the key of F)
or
Bb / C-E-G (C7 chord, same other stuff as above)
F / C-F-A (resolves to 4 of C, which is F)
D / D-F-A (D minor chord)
D / D-F#-A ( D major chord, F becomes F#, which is the leading tone in the key of G)
or
C / D-F#-A (D7 chord, same other stuff as above)
G / D-G-B (resolves to 4 of D, which is G)
Bb / Bb-Db-F (Bb minor chord)
Bb / Bb-D-F ( Bb major chord, Db becomes D, which is the leading tone in the key of Eb)
or
Ab / Bb-D-F (Bb7 chord, same other stuff as above)
Eb / Bb-Eb-G (resolves to 4 of Eb)
You see what I mean? Key is really irrelevent for where a dominant chord or a dominant 7th chord wants to resolve to cuz that resolution chord ain't always 1. The reason why I say "wants to resolve" because depending on the key you could resolve to 3 or 6. Here is an illustration using one of the above examples:
C / C-Eb-G (C minor chord)
Bb / C-E-G (C7 chord, Eb becomes E, which is leading tone in key of F)
F / C-F-A (resolves to 4 of C, which is F)
or
*A / C-F-A (resolves to 6 in the key of C or 3 in the key of F)
*Notice that the RH part of the chord is still 4 in the key of C, but since there is an A in the bass, it changes the entire chord to 3 or 6 depending on what key you in. In other words, it is an inversion.
You see that? The resolution chord can be so many things other than 1, but it can also resolve to 1 or 5. Let me show you:
C / C-Eb-G (C minor chord)
C / C-E-G (C major chord, Eb becomes E, which is leading tone in key of F)
*C / C-F-A (resolves to 1 in the key of C or 5 in the key of F)
*Notice that the RH part of the chord is still 4 in the key of C, but since there is a C in the bass, it changes the entire chord to a 1 or 5 depending on what key you in. In other words, it is also an inversion.
Now, I have just showed you that a dominant chord can resolve to 1, 3, 5, and 6. You can also keep that secondary dominant principle going on forever if you wanted to. Here it is:
C / C-Eb-G
C / C-E-G or Bb / C-E-G
F / C-F-Ab
F / C-F-A or Eb / C-F-A
Bb / Db-F-Bb
Bb / D-F-Bb or Ab / D-F-Bb
Eb / Eb-Gb-Bb
Eb / Eb-G-Bb or Db / Eb-G-Bb
Ab / Eb-Ab-Cb
Ab / Eb-Ab-C or Gb / Eb-Ab-C
Db / Fb-Ab-Db
Db / F-Ab-Db or Cb / F-Ab-Db
Gb / Gb-Bbb-Db
Gb / Gb-Bb-Db or Fb / Gb-Bb-Db
F# / F#-A-C#
F# / F#-A#-C# or E / F#-A#-C#
B / F#-B-D
B / F#-B-D# or A / F#-B-D#
E / G-B-E
E / G#-B-E or D / G#-B-E
A / A-C-E
A / A-C#-E or G / A-C#-A
D / A-D-F
D / A-D-F# or C / A-D-F#
G / Bb-D-G
G / B-D-F or F / B-D-F
(repeat, you are back to where you started from)