In your practice you have probably encountered songs that have chords that are not in the key. As mysterious as those chords may seem, they can often be identified easily, once you know some of the common ways that harmony moves out of and back in to a key. This lesson will explain some of those ways.
When there is a modulation in a song, the harmony settles in to a new tonal center or key, where it may stay either for a period of time or for the remainder of the song. If you feel that this is happening in the song you are working on and you are looking for the new key, suspect a movement to a nearly related key (p. 49 in A Guitar Player’s Guide to Music Theory). If the new key is not a nearly related one, listen for is a key that is either a whole-step or a half-step from the original key. There are, of course, many other possibilities, but these are common, so try them before you do too much hunting.
Often, there is no new tonal center that the harmony settles into for a period of time. The song essentially stays in the same key, but has one “ mystery chord.” A common example of this type of harmony is the use of a secondary dominant chord.
The secondary dominant is easy to understand, once you are clear on the term “dominant.” The dominant chord in the scale harmony is the V or V7 chord, so in the key of C, G or G7 is the dominant chord, in the key of G, D or D7 is the dominant and so forth. The secondary dominant is the chord that is dominant in the key of a chord that occurs in the original scale harmony. In the key of C, for example, the V7 of ii (Dm) is A7. (A7 is the V chord in the key of Dm.) The V7 of the iii chord (Em) is B7, The V7 of IV is C7, the V7 of V is D7 and the V7 of vi is E7.
The secondary dominant for you to become familiar with first is the V7 of V chord (written V7/V), as it is the one you will probably encounter most often. Here is a list of those chords in some common guitar keys:
Key of C: V7/V = D7
Key of A: V7/V = B7
Key of G: V7/V = A7
Key of E: V7/V = F7
Key of D: V7/V = E7
Practice finding the V7/V chord in other keys and then begin working with the secondary dominants for other chords in the scale harmony.
Be patient and diligent as you learn to find chord progressions by ear. You will continue to improve with practice and it is a truly rewarding and satisfying skill to possess!