What are weak and strong chords?

Question

I'm reading Jazz Theory by Stuart Smith. In the chapter on functional harmony the author introduces the notion of strong and weak chords:

I, II, and IV are generally treated as strong chords. V, VI, and III are generally treated as weak (IIIm7 and VIm7 as representatives of the Tonic constitute a special case, which is discussed below.) As we shall see, VII has a context-dependent role in tonal music. Sometimes it is treated as a form of dominant, while at others it is used as part of a circle- of-fifths structure such as IV VII III. In either case it would be considered a weak chord in the sense used here.

Please could you explain what does it mean - strong and weak chords, and what qualifies a chord as strong or weak? Is it somehow connected with stable and unstable degrees they are made up from?

Answer 1

This can be subjective to some extent, and even your book says "in the sense used here".

But in general, based on what I learned directly from jazz musicians over the years, the consensus is that:

1. In a tune, in a sequence of chords, some of the chords are like milestones, feel like major landmarks, and changing or missing them would completely change the overall harmonic sense. Other chords could be skipped or altered without doing too much damage to the original feel. You may call the former chords "strong" and the latter "weak". And if you analyze them harmonically, you will find that these "strong" chords usually correspond to those listed in the text.

2. Another way of looking at it is to consider how strongly does the chord implies what melodic lines feel "in" or "out". In this sense, for example, a I or a II chord will strongly dictate the melodic sense (major and minor modes). While a III or VII chord will comparatively allow the melody to be much more all over the place -- until it eventually resolves on another strong chord.

(And the last point can be yet another way of defining a strong chord: a chord on which you can resolve a melodic line; while a weak chord is one over which you cannot satisfactorily resolve a melodic line, but must continue toward a subsequent strong chord)

Answer 2

Here is another quote from the book...

... In a context where the harmonic rhythm is the half note, the first (strong) half of each measure is occupied by I or II, while the second (weak) half is occupied by V or some other weak chord.

When he speaks of the first/second halves of measures and strong/weak, it actually means strong/weak metrically. The first beat of bar is considered metrically strong and certainly the last beat of a bar in any meter would be considered metrically weak.

When you continue reading the text he explains some chords are found typically in a (metrically) strong or weak position. That intended meaning may not be clear, because he uses wordings like "weak chord" rather than "metrically weak chord. Nevertheless, he says the V chord would be "weak" - meaning used in a metrically weak position - and the I chord would be strong - meaning used in a metrically strong position.

Let's look at the ii V I progression from that point of view...

 x ii | V I

...would be uncharacteristic, because it places the V in a strong position on beat one after the barline and the I in a weak position.

 x ii V | I

...would be the characteristic position with V in the weak position before the barline and I in the strong position on beat 1 after the bar.

I think this is a good rule of thumb, but it won't always be true.

Certainly it is good to understand harmony in terms of metrical positioning and how that effects phrasing.