My private students often ask me why certain chords work together. The question is a GREAT question and leads any thoughtful musician down some very interesting roads. There is a very simple reason why certain chords work together, but the answer to this question also illuminates ALTERNATIVE chords that we can use. That's really the topic this article (and YouTube video).
Why Chords Work or Don't
Chords (which form harmony) are made from notes - typically a triad. A triad implies a scale and a competent musician benefits from knowing their scales (just as a competent writer benefits from know the alphabet). A triad implies a scale because a triad always represents a set of boundaries or constraints. A triad is the Root, 3rd and 5th... of what, you ask? Of a scale! So, you can't have a triad without a scale. One implies the other.
Another way of describing how harmony works is to look at how a triad IS the Root, 3rd and 5th of one specific scale (or occasionally a "mode"). We must get our hands on that scale to start really making sense of why and how our harmony is working. For example, a D major triad is made up of the Root, 3rd and 5th of a D major scale. Its that simple. In the key of D major, we'd call a D major triad a I (roman numeral one) chord. It means that if we build a triad from the Root (or first degree) of the scale, we get a D major triad - D, F# A.
When we build a chord progression out of chords that ALL contain notes from the same key, we'll find that the chords "work" together. If we build some of the chords from notes that aren't in the key, then they won't work.
What Are Alternative Chords?
Alternative chords are still made from notes in the key, but they are built relative to different roots. This gives us different "colors" to work with. A moment ago I explained that a D major triad represents a I (roman numeral one) chord because it is built from the 1st note of the scale (a.k.a. the Root). But, we can build ii (roman numeral two) or vii (roman numeral seven) chords. When we do, the chords will still "work" together, but they'll be offer different sonic personalities. Check out this video explaining a basic harmonic progression example in the key of D major:
Next, check out the next part in this series: Playing Cooler Chords Part 2.