|What chord to use?
Principles of good root movement
What is root movement? Root movement is identifying what chord is moving to what other chord, regardless of inversion. In other words, it is what Roman numeral is moving to what other Roman numeral. For example, when the tonic chord moves to the first inversion supertonic chord (I – ii6), the root movement is up a step (from I to ii). Notice that root movement is not at all the same as the movement of the bass voice. It only relates to the roots of the chords in the progression.
There are three very common root movements that are consistently found in progressions in tonal harmony. They always sound good, they move the harmony forward, and they also work well to provide resolution for sevenths of chords. Learn these three, and they will help you know what chord to move to next, or what chord might make a good chord to approach any given harmony. Just remember that the inversions of the chord play no part in the measurement of the root movement, only the Roman numerals.
In this description, I am choosing to use the “shortest” distance to describe the type of movement, since I have found that for most students this works most reliably.
The three most common movements are:
Up a fourth: (could be described also as down a 5th, but for most students, it is easier to accurately count letters upward rather than down.) This is the strongest root movement – like V to I. Most of the time the interval is perfect, but not always since there is one tritone in every scale. Examples: V – I, ii – V, vi – ii, iii – vi, I – IV, III – VI, VII – III, IV – Vii, etc. Remember that this root movement just gives you the Roman numeral of the chord that is to be used; you may add inversions as you please to either or both of the chords. When using this root movement with root position triads, there is always one common tone, and the other two notes can move by step.
Down a third: This is the simplest and smoothest of the root movements, since only one note is required to move in diatonic progressions, since there are two common tones. Because of those common tones, the second chord does not sound all that much different than the first. Examples: IV – ii, I – vi, viio – V, VI – iv, etc.
Up a step: when describing this one, we mean up to the next letter name or step in the scale; it can be either a half step or a whole step. This type of root movement is the most difficult for voice leading, since all three notes must move, and it is very easy to end up with parallels. Use contrary motion to avoid this. Examples: V – vi, IV – V, I – ii, etc. Again, inversions can be used as needed, which also may help avoid parallels. For example, try using I – ii6 instead of I – ii, if parallels are a problem.
Add to the above: I (or i) goes anywhere. In other words, the tonic chord can go easily to any other chord in the key, regardless of root movement.
Use this when resolving seventh chords: the three good root movements (up a 4th, down a 3rd, up a step) will always provide you with the note needed to resolve a seventh in a chord. For example, if you are using a ii7 chord and want to know what chord would be good to put next, think about the step of the scale this is the seventh of the ii7 chord (the tonic note.) Since sevenths of chords need to resolve down by step, the chords following the ii7 needs to contain the note one step down from the tonic note, which is the leading tone. If you test the three root movements described you will see that they each contain the required note:
Up a fourth from ii is V, and it contains the leading tone.
Down a third from ii is viio, and it contains the leading tone.
Up a step from ii is iii, and it also contains the leading tone.