Transition Chords

Do you mean like to switch keys? Cause if so then that is a more specific question. If your staying diatonic (which is most likely what you mean) then its more a matter of learning how each chord functions then having a specific transition chord. Check out this theory website http://www.musictheory.net/ and look the section under "lessons" called "common chord progressions." This gives a very good explanation of how to make a good chord progression and consequently a good song. Theres a lot of good basic info on there if your lacking in your theory knowledge.

If you meant to switch keys then there is no one magic chord, it depends what key your in and what key your switching too.

Also I did this and it helped me a lot. Before writing your song write out all 7 diatonic chords for whichever key your song is in. Then play the 1st (in C this would be C) and then play another chord, say to start the 5th (in C it is G). Then underneath the G chord write out how that made you feel, theres no wrong answer mine were things like "im jumping off a waterfall" just something so YOU will mentally identify the chord. Do this for all other diatonic chords and when your writing your song youll find yourself knowing in your head which sound you want next, and if you look at your descriptions youll be like "of course my dog just got hit by a car! 6th minor yes!"

The dom7 chord of the key you're moving to will help the transition. If you're in the key of G and you want to go to A, then playing E7 will signal the shift (assuming you'll start the progression for the new key on A).

I think what you're looking for is called 'pivot' chords. There are a few different kinds of those...

1. There are chords that appear in more than one key. if you're in the key of E, and you want to move to the key of C, you can do it in a bunch of steps...

E chords: E, F#m, G#m, A, B7, C#m, D#º
C chords: C, Dm, Em, F, G, Am, Bº

none in common. But you can step through other keys on your way...

E to A:
E chords: E, F#m, G#m, A, B7, C#m, D#º
A chords: A, Bm, C#m, D, E7, F#m, G#º

so you could play in E until you hit an E chord, and replace it with E7, or you can play until you hit an A or F#m chord, and continue with chords in the key of A from there. Then you could move from A to the key of D, then the key of G, and finally A using the same logic... so your progression might end up:

E-A-B7-E7-A7-D7-G7-C-F-G etc.... this is called modulation by secondary dominants

2. Certain chords, like augmented and diminished chords, are 'symmetrical' - they can be spelled with any note as the root. So if you use a diminished 7th instead of the diminished triad, you can go more directly... in E, the D#º7 is spelled D#-F#-A-C. You can jump from that directly to the keys of G, Bb, or Db - since their diminished chords (F#º7, Aº7, and Cº7) have exactly the same spelling - the chord notes appear in both keys at once. That could give you something like:

C-Bº7-Dm-Gm-Cm-Eb7-Bb... here the viiº in C would normally move to the iii (Em), but by moving to Dm instead (the iii in Bb) we hear it as a key change. You can actually look at this one both ways - the Dm chord appears in both keys!

3. You can pivot by substitution. If you look at a G7 chord, G-B-D-F, you'll see that the upper three notes are a Bº triad. So instead of playing G7, you could play Bº7 (the same chord as a G7b9 without a root) - and that's exactly the same chord as a Dº7 (key of Eb), Gbº7/Fº7 (key of F#or Gb), or Aº7 (key of Bb). Then you end up with something like:

C-F-G7-Bº7-Bb7-Eb... here the G7 moves easily to Bº7 (they're only different by one note), and the Bº7 moves just as easily to Bb7 (again, only different by one note).

4. You can also change keys without a pivot chord. It's common for music to simply move up by a half step or whole step (you can move down too), and in the right context you can simply leap from one key to another and it'll work as well - that just slaps people in the face with a new key.

2. Certain chords, like augmented and diminished chords, are 'symmetrical' - they can be spelled with any note as the root.

Could you give an example of how to use an augmented chord to modulate to another key? All I ever used them for is as passing chords, ie. C - C+ - C6 - C7 - F. To what chord does an augmented chord want to resolve to anway?

Because of the nature of augmented chords (each note is two full steps (a major third) away from each other, each individual augmented chord actually is three different ones, as Tom point out. For example:

Caug = C, E, G#

Eaug = E, G#, C (which is B#)

G#aug = G#, C (B#) and E (D##)

It's like having a door to three different keys.

Typically, augmented chords will resolve either to the IV chord or vi. Occasionally back to I. Using the same example, Caug would normally go to F, Am or back to C. This isn't (as are most things in music) set is stone, but it's a good bet.

We've got some good articles on the use of augmented and diminished chords on our theory page, https://www.guitarnoise.com/theory.php , if you're interested.

Peace