Any two different chords in sequence constitute a harmonic progression. The relationship between the two chosen chords can be quantified according to its strength, where stronger progressions sound more “convincing”. However a progression containing only strong relationships can sound boring and predictable, so you should not treat the terms “strong” and “weak” as perjorative, you need to balance both types in your compositions.

Given that a triad has only three notes, there are only three basic relationships between any two triads:

1. The triads have no notes in common.
2. The triads have one note in common.
3. The triads have two notes in common.

Of course if the triads have all three notes in common they are the same so that doesn’t count.

In any given key, triads with no notes in common can only occur when the roots are a second (or a seventh) apart. They are said to be in second relation:

These are the strongest progressions, but not the most important, since they are somewhat too strong for continuous use.

Chords in any given key with only one note in common can only occur if the roots are a fourth or a fifth apart. They are said to be in quartan (4) or quintal (5) relation:

These progressions are the most common, because they include both the authentic and plagal cadences.

Chords with two notes in common, in a given key, can only occur if the roots are a third (sixth) apart, in which case they are said to be in tertian relation:

These progressions are considered relatively weak, but remember that weak does not mean undesirable.

Beyond the basic classification above, we can further categorize progressions into whether they are ascending or descending. Simply put, an ascending progression is when the root of the second chord is not in the first cord. Ascending progressions are considered stronger. Descending progressions then, are those where the first chord contains the root of the second and are considered relatively weak.

We can therefore describe the relationships between two chords any interval apart as follows:

second

second relation, ascending (super strong)

third

tertian relation, descending (weak)

fourth

quartan relation, ascending (strong)

fifth

quintal relation, descending (relatively strong)

sixth

tertian relation, ascending (relatively weak)

seventh

second relation, ascending (super strong)

The idea that certain notes have a natural tendancy to move to others is called voice leading. So far we have only seen the leading note rising to the tonic and the seventh of the Dominant Seventh falling to the third of the tonic. There are a few others and if you want your music to flow, you would be wise to let these notes do what they want to do.

Music theory, for the most part, deals with the interaction of four separate “voices” called Soprano, Alto, Tenor and Bass (S, A, T, B). They are written on two staves, the Soprano and Alto in the upper stave with a treble clef, and the Tenor and Bass in the lower stave with a bass clef. Their ranges are roughly as follows:

Although the ranges overlap, it is conventional that for any given chord the four voices stack with the Soprano on top, the Alto below the soprano, The Tenor below the Alto and the Bass on the bottom. Here’s an example of four part harmony:

That very simple progression demonstrates quite a lot of the rules of harmony, rules that you need to know even if you intend to break them later on. I need to lay down one really important rule first, then we can examine that progression in more detail. The rule is:

• Avoid parallel octaves and fifths at all costs.

Parallel octaves and fifths occur between two voices, an octave or a fifth apart, when they both move in the same direction by the same interval. They are especially evil when they occur between the Soprano and the Bass, but should be avoided in all cases.

Now back to our progression. First of all notice that there is often quite a gap betheen the Bass and the Tenor. This is normal and desirable to get a convincing bass line. Let’s discuss each chord in turn, and consider its relation to its neighbours.

I

The tonic chord in root position. Notice that the Soprano, Alto and Tenor notes are spread out rather than close together. This is called open position. Their positions are not arbitrary though: they occur as E, G and C (top to bottom) This is the same order as they would appear in a first inversion triad read bottom to top. Put another way, you can get from open to close position (where the notes are as close together as possible) by transposing the Tenor up an octave. Another thing to note is that it is only the Bass of the chord that counts when identifying the inversion of the chord – the S, A and T can appear in any order.

V⁶₄ (open position)

While root position and first inversion chords can be used pretty much anywhere, the second inversion has more limited uses, and this is one of them. It is called a passing 6-4 and is placed so that its bass is on the note between the bass of a root position chord (I) and its first inversion (I⁶).

I⁶

Here’s a basic rule: don’t double the third in a major 6-3 chord. There is a reason for this. Any major chord really wants to be a dominant, in which case the third acts as the leading note and wants to rise by step to the imagined tonic in the next chord. This tendancy is most pronounced if the third is in the Bass. If you were to double the third, you would have two thirds both wanting to rise by step and parallel octaves would result. For that reason what might have been an E in the Tenor has been moved down to the C, so the chord is neither in open nor close position.

IV (close position)

This chord is often used to prepare an authentic cadence (V–I or I⁶₄–V–I).

I⁶₄ (close position)

This is the other main use of a 6-4 chord, as a cadential 6-4 where it preceeds the dominant in an authentic cadence. This highlights why 6-4 chords are more awkward to deal with. They behave like a double appogiatura where both the 6 and the 4 want very much to resolve downwards by step to the 5 and the 3 on the same bass. I⁶₄–V does just this. Unless cleverly hidden (such as in a passing 6-4) almost any other chord following a 6-4 will sound wrong in some way. Also notice how the progression I V⁶₄ I⁶ IV I⁶₄ forms a scale. A smooth Bass is a sign of good four part writing.

V⁷ (close position)

Since it follows a cadential 6-4 it has the same bass note, but here the bass drops an octave for a more final effect. Notice that the third (B) rises to the tonic (C) while the seventh (F) falls to the mediant (E) in the next chord. Again, to allow these natural progressions, the following chord is in neither open nor close position.

I

As discussed, the progression V–I at the end of a phrase constitutes an authentic cadence. If furthermore both chords are in root position, and the Soprano of the I is the tonic, then this is a perfect cadence. Many people mistakenly refer to any authentic cadence as a perfect cadence, but there is a difference.

Don’t be fooled into thinking that this is just theory – good four part harmony can stand on its own as a finished piece of music, and it forms the skeleton of most if not all tonal music that has ever been written.

If you add the seventh from the root to a triad, you get a seventh chord:

Different chords of the scale have different sevenths. For the major scale, they are

Notice that V⁷ is the only major chord of the key with a minor seventh. This means it is a defining chord of the key. In fact since it contains both the fourth and the seventh degree of the scale, this so-called Dominant Seventh unambiguously defines the key. For example in the key of C, the Dominant seventh consists of the notes G, B, D and F. The B♮ means we cannot be in a flat key, because B♭ is the first flat, and the F♮ means we cannot be in a sharp key, because F♯ is the first sharp.

The Dominant Seventh is the most common seventh, and the progression of dominant seventh to tonic is the strongest progression in Western tonal music:

You may be thinking Chord vii also contains those notes, and you’re right, however if we choose the diminished seventh of vii (which usually sounds better than the minor seventh) then we have a chord completely composed of consecutive minor thirds, and that chord can be the vii^o7 of four separate keys (this is useful in modulation, discussed later).

Because there are four distinct notes in a seventh chord, there are three inversions, as well as root position:

Using I as an example, the root position is just written I⁷, as the 3 and the 5 are taken as read. Likewise the first inversion is I⁶₅, the second inversion is I⁴₃, and the third inversion just I². There is a useful mnemonic sequence to remember these: 7, 6-5, 4-3, 2.

The notes of a triad can be rearranged so that different notes are in the bass. These are called inversions.

When the first note of the triad is in the bass, like all the chords we have seen so far, the chord is said to be in root position:

This chord, if it is the key of c major, can be written I⁵₃, meaning it is the notes of the tonic chord arranged so that there is a fifth and a third from the bass. But since thirds are taken as read, it is more usually written as just I.

If the third of the triad is in the bass, we have what is called a first inversion:

Because this chord has a sixth and a third above the bass, it can be written in full as I⁶₃, but again thirds are taken for granted so it can be abbreviated to just I⁶.

Unsurprisingly, if the fifth of the chord is in the bass, we have a second inversion:

This chord has a sixth and a fourth above the bass and is written I⁶₄, no shorthand allowed. I guess it could be written I⁴ with the third above the fourth implied, but I’ve never seen that.

This odd convention of a stack of numbers alongside the roman numeral of the root may seem unnecessarily complicated, since it is saying “the notes of such and such a root triad rearranged so that these intervals are above the bass”. They actually come from an old style of musical notation called figured bass where the composer would write out the melody and the bass line in full, but would then write these numbers under the bass line to indicate to the keyboard player which chords they should be playing. In thatcase the numbers (without roman numerals) did indicate the intervals above the given bass note. But when that numbering was carried over into harmonic theory, the root of the chord was considered (rightly) much more important than the bass note of the chord, and so we have this rather counter-intuitive naming system that you just have to get used to.