Harmonic stability

Harmonic stability refers to how "at rest" the harmony feels. This concept is closely tied to, and heavily influenced by, harmonic function. A tonic chord is often described as "feeling at rest" while a dominant chord is often described as "unstable" or "needing to move (back to the tonic)".

Another factor that influences harmonic stability is inversions. The most prominent example is that a $I^6$ chord feels much less stable than a $I$ chord, and in a Classical context, $I^6_4$ actually has dominant function (see: cadential 6-4).

A surefire way to destabilize the harmony is to use a sequential progression.¹ It's worth noting that, while these progressions would be useful in sentence theme types, they are not very common as they tend to take up too many measures to be executed.²