When you first learn music theory, you learn to paint within the lines. This means you learn rules to follow. And when you understand and can use those rules well, you realize that they weren't rules at all but guidelines.
Music theory is designed to serve us, not the other way around. So, when you learn what chords are within the key, this does not mean there aren't any other options. It just means that those chords will always work together well.
In this article, I want to share the top 2 most common ways songs go outside the key. If you ever want to transcribe songs that do this, understanding what is going on is the first step. So, let's get into the first way songs go outside the key.
1. Borrowed Chords:
Borrowed chords are simple when you borrow a chord from the parallel key. Parallel keys share the same root note but change between major and minor. For instance, the parallel key of C major is C minor.
In the key of C, there are chords: C Dm Em F G Am Bdim
In the key of C minor, there are chords: Cm Bdim Eb Fm Gm Ab Bb
If I have this chord progression in C major: C Am F G. I could replace any chord with one from the parallel minor, such as C Am Bb G.
This works both ways; I could be in the key of Cm and borrow from the parallel major.
2. Secondary Dominant Chords:
Secondary dominant chords, on the other hand, involve introducing dominant chords that are not within the key.
In a major key, the dominant chord is the fifth chord (V), which resolves to the tonic (I). A secondary dominant, however, introduces a dominant chord that points to a chord other than the tonic. For example, in the key of C major, the G7 chord is dominant. By introducing a D7 chord (the dominant of G) creates tension that resolves to the expected G chord.
To do this, look at any chord within the key and go up a 5th. This will show you what secondary dominant chords you can use.
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