Chord progressions can be tonally strong or ambiguous. I like to call the ambiguous ones “fragile” for a reason that I’ll describe a little further on. But to illustrate the concept of strong versus fragile, let me use an analogy. Imagine you are looking at a map of your city. You place a dot on your house, and then you place a dot on a store 3 miles away. Then you draw lines that follow every possible way back home from the store. The most direct routes (the ones that make the fewest turns) would be the “strong” ways. The ways that meander a bit, where it’s not absolutely clear where you’re headed, could be termed the “fragile” ways.
This is the best analogy I can come up with to describe the difference between strong and fragile progressions. In a nutshell, strong progressions are ones that strongly indicate the key you’re in (i.e., they point clearly back to the tonic chord), and fragile progressions are the ones that meander a bit, and don’t clearly point to any specific chord as the tonic.
For example, starting a musical phrase with: C F G… makes it clear that C is serving as the tonic chord. Those three chords are only usually found in C major, and thus there is no harmonic ambiguity.
But starting with: Em F Em Am… is less clear. Those are chords that one can find in C major, but you can also find them in A minor (natural minor). And the harmonic function is less clear. It’s this ambiguity that I’ve been calling “fragile”.
So armed with that analogy and those examples, you can start to see that a progression being strong or fragile is a matter of degrees. If a progression can only naturally or easily exist in one particular key, that progression will be strong, particularly if the progression focusses on tonic-dominant movement. The more possible keys a progression could possibly point to (regardless of the key you’re actually using), the more “fragile” it becomes.
I like the term “fragile” because since these particular chord progressions are a bit tonally ambiguous, they could be used to help “break” out of the present key to move into a new one.
This sense of fragility of a progression should never be equated with something musically weak or undesirable. Most songs use fragile progressions, particularly in the verses, as those are the kind of progressions that partner well with the kind of lyrics one uses in a verse.
