Four part chords (harmony) are sometimes referred to as "Block Chords". This type of harmonic structure, unlike the 2 and 3 part harmony, is the first that is capable of defining the complete harmonic structure of chords beyond the triad. Block Chords have a special sound which is often associated with older styles of American music, such as the sound of big bands and tight vocal groups from the earlier days of jazz that appear after dixieland. When we look at Block Chords, we have a few options in how we voice them. We can voice them as straight block chords, or we can open them up a bit by dropping one or two tones down an octave. Let's have a look at these voicing options:
Block Chord Voicings for a CMaj7
In the first example (Straight Block Chord), we have written (voiced) the chord in it's root position. If we want to "open" this chord up a bit, we can voice it using the Open Block Chord #1 or #2 options. In the Open Block Chord #1, we take the the 2nd note from the top and drop that down an octave, in this case it is the G. Now the G is in the bass clef. If we want to open it up a bit more as in Open Block Chord #2, we can also drop the 4th note from the top an octave, in this case C. Now both the G and C are in the bass clef.
Your decision on when to use any of these three (3) voicing of a 4 part chord can come about do to stylistic choices, to accommodate instrument ranges (keeping each instrument within the notes it can play), or to create counterpoint in the outer voices (top note and bottom note).
Block Voicing Working Procedure
Let's create a melody using notes that are chord tones from our CMaj7 chord, then voice that as a straight block chord. The way we voice a Straight Block Chord from a melody line is pretty easy and straight forward. In this example, we are using a CMaj7 chord as the harmony, so we simple spell the chord down from the top note using each chord tone.
The first top note is B, so we spell the chord down using CMaj7 in this example (see chord spellings). Since we are harmonizing this example using a CMaj7 chord, we spell the chord down from the top note. We start with a B in the top note (the Maj 7th) and spell the chord down from there. Same with the E in the melody. We are just spelling the chord down from the top note using the next chord tone every time. The next chord tone under E is C, the next under C is B, and so on. We continue this to the end.
Now we have our melody note harmonized in Straight Block Voicing. This is a perfectly acceptable way to voice this passage in 4 part harmony. Now let's voice this same thing using the Open Block Voicings discussed above. We are doing the same thing, except this time we are dropping either the 2nd note or the 2nd & 4th notes down an octave to create a more open sound. Open Block #1 drops the 2nd note from the top down an octave. Open Block #2 drops the 2nd & 4th notes down. I have left the last chord in Straight Block voicing (although not the best voice leading, good visual for our example).
While our example is all harmonized using a CMaj7 chord for clarity, this way of harmonizing a melody note into the various block voicings would apply to entire passages making use of several chords, including passing chords.
When Melody Note is the Root of a Maj7 Chord
When we have a melody note that is the root of a Maj7 chord, we usually want to alter the chord from a Maj7 to a Maj6 chord. The reason being is we want to avoid the next note down from the top melody note being a half step (C to B in the CMaj7), and more importantly, we want to avoid the minor 9th interval that would occur when using the Open Block Voicings. Other than a V7(b9) chord, a minor 9th interval sounds ugly and rubs the wrong way. We cure this problem when the melody note of a Maj7 chord is the top note by replacing the Maj 7th (B) with the Maj 6th (A).
Play these examples on the piano and you will quickly hear the difference. The 2nd voicing of each chord is much more pleasant to the ear when we substitute the Major 6th for the Major 7th when the root of the chord (C in this example) is the melody note.
Five, Six, or More Part Chords Using 4 Part Block Voicings
When we have a chord that has more than four tones, for example a CMaj9, we simply voice down from the top note in the same way. We voice down from whatever the top note is. For example. Let's say instead of our CMaj7 chord, perhaps we have a CMaj9 chord a D (the 9th) in the melody. We voice the chord down using the same approach as before using the next chord tone down. Naturally since we have a higher extension, we will be leaving a note out. This works just fine because we are still defining the chord by using the upper extensions (9th & 7th in this example). Let's have a look at this:
We are still voicing this down from the top note using 4 part harmony. We omit the C in this case as the C would most likely be covered by the bass instrument or left hand of the piano. If you look at it a little closer, you will see what we really have is an Emin7 chord voiced on top over a C in the bass. Together they make up a CMaj9 chord.
So why not just make it a 5 part chord? We can if this is being played by a piano or 5 instruments. But what if your instrumentation is 4 saxes? We need this technique when harmonizing 5 or more part chords, using block style voicings, for only 4 instruments.