You might think that when I talk about “strong” progressions I’m really talking about “good” ones, but that’s not what I’m referring to. A strong chord progression has a particular set of characteristics that contrast with the other category: fragile progressions.Good songs use a combination of strong and fragile progressions.
So what is the principle regarding strong progressions? Here it is:
Two chords that have a note in common will form a STRONG PROGRESSION; And if that first chord moves up by four notes or down by five notes to reach the next chord, the progression becomes even stronger.
So let’s take the first part of this principle. When two chords have a note in common, it often strengthens the ability of the first chord to move to the second one. How strong a progression is really depends on musical context. There’s no good way to put a “number” on how strong a progression is. For example, two chords might by themselves form a strong progression, but that progression can be weakened by the presence of other chords in the vicinity. But if the two chords have a note in common you can be assured that they’ll have at least some measure of strength.
Add to that the second part of the principle, which states that the progression becomes even stronger if the roots of those chords are a 4th or 5th apart. So the progression C F is very strong, not just because they share a common note (C) but that their roots are a 4th apart.
Keep in mind that progressions are usually not just two chords long; they are often 3, 4 or more chords all brought together in one longer set of changes. So progressions need to be evaluated for their strength by taking the entire progression into consideration.
So how do you use this knowledge? It’s not that difficult. Page 84 from “The Essential Secrets of Songwriting” e-book lists many short progressions, and evaluates them for how strong or fragile they are.
What you need to consider is that good songs use a combination of strong and fragile progressions. (Remember, a fragile progression does not mean that it’s bad, and most songs use them.) A verse melody accommodates fragile progressions better, because verse lyrics are almost by definition narrative and “inconclusive” in style. A chorus melody usually requires stronger progressions, because the chorus lyrics are more conclusive and reflective.
-Gary Ewer, from “The Essential Secrets of Songwriting” website