Here is an overview of my six steps for creating four part harmony parts from a given melody. The arrangement in this case if quite traditional – this is for the sake of clarity and introducing fundamental concepts to people just starting out. If I dropped crazy stuff right now, I’d have fun but no one would learn anything
I have used a Bach melody to illustrate the steps – and the harmony is kept pretty much in line with the historical period.
1 – Understand/create the source material
Whether we are writing a melody ourselves, or working from source material, the first step is to analyse the material so we can start to formulate the starting and finishing keys, and the “journey” we want to the harmony to take between them.
Here’s the melody:

Listen to the audio here:
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Many people will say to work out the final cadence first – but that is a bit clinical for me, I prefer to get an idea of the key first, and if it changes throughout the section we are working on. I always try to think of it as a story that goes from one place to another. If the key is the same at the start and finish – we might want to think about making the journey the interesting part!

So I hit the piano for a bit and tried the melody in the most likely keys – C and a minor, and to be honest it’s a bit ambiguous. So I’ve opted to start in C but keep the option open to switch to a minor – but I’ll definitely end in C. Those repeated e’s are a problem as the harmony could go anywhere with those. But I’m going to be sensible – and see how they look once we’ve got more harmony in place.
2 – Work out the chords we want to use:
The next step is to “join the dots” between the first and last chords, and create a sketch of what chords will complement the melody best, as well as adding to a sense of movement.
This sketch does not have to be fully detailed or set in stone – it will likely change when we write the other voices.
In this case I’ve gone with the chords Bach used ( not because I can’t do it, but because it makes for a good example this way! You’ll see why later on…):

You may have noticed the E major chord in there – this is the moment that stands out as a key change – the rest is essentially in C, but that E major chord contains a G# which is very much part of the a minor tonality – that is the only moment where Bach states unequivocally - we are in a minor!
But straight after that we are back safely to C major and our final cadence. The key change leading into the E is a surprise but the transition back is smooth because first two chords after the E major, are in both a minor and C major – so there is no sense of a jump on the way back.
We have almost finished fleshing out the chords – there are jus a couple of finishing touches we can make to make it more interesting and add a little detail:

There is now a little Dm-G progression over the d note before the middle and end C chords. This is just to create a cadence and add a litte movement instead of the one-chord-per-bar feel that had gone before.
At this point, we can check how the harmony supports the melody(this is just block chords, not any real arrangement, just to give you a sense of how the harmony moves from start to finish):
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3 – Create the outer voices
Assuming the melody is given, and a rough sketch of the chords is written out, then next we need to create a dynamic bass line.
The best way to do this is to take distinctive elements from the the melody and either echo, compliment or contrast them.
There are some key points to remember here:
The outer parts stand out and create the overall shape of the music – make them interesting.
The bass line should clearly present the harmony when played just with the melody.
The bass line should complement but also contrast with the melody
It sounds pleasing when the outer parts move in opposite directions.
Avoid moving these parts in parallel octaves, unisons and fifths.
Avoid extreme high and low notes – this is not just when writing for voices, the chorale style tends to sit in a comfortable range. Also, as the voices tend to be written relatively close together, going very high or low risks overlapping with another voice – but more on this in the next step.
In fact, in our case it is fine to start with the same rhythm as the melody while we play with the harmony:

This is ok but a bit simple and dull…we can add a few details to make it a little more interesting, bearing in mind the points made above.
We have added plenty of opposite (or contrary) movment, avoided parallel ocatves, unisons and fifths, and added some runs that elaborate on the harmony during the key change.

Listen to how the harmony sounds essentially as clear to the ear as it did with the full chords in the example above:
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4 – Sketch the inner voices
This entails filling out “by numbers” what the harmony should be according to your list of chords. When you have the harmony chosen in advance, there are only a few options as to what notes you can choose for the inner voices.
The idea is to find a straightforward route through the harmony for both inner voices. In the traditional chorale style, the inner voices should ideally move step-wise and not drawn attention to themselves – their job is to fill the harmony and blend well, not stand out:

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5 – Add details and finalise the inner voices
After you have found a good line for each of the inner voices, you can start to embellish them, and check that they work with the outer parts.
6 – Finishing touches/proof reading
This is simply a case of checking for mistakes, and going over the finished product to make improvements, change choices and generally taking a second look at everything you’ve done so far and evaluating your work.
In this case we can add a few extra notes to the inner parts to create some runs that pre-echo the key change runs in the bass – plus move in the opposte direction. This creates a sense of unity – that similar elements are being used throughout the piece, meaning recognisable shapes keep cropping up for the listener.
So here is the final version as Bach would have it:

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Now this is the point where you’ll see why i picked the Bach arrangement. I’ve done my own little arrangement – and in no sense do I think it’s great or better than Bach or anything like that. This one has simply been done to take things a bit further – a lot more detail and ornamentation – the harmony is slightly expanded (but not much) and the rhythms have been developed into some new variations.
I leave you to have a look at the changes I made (and judge how tasteful they were, heh) and see if you prefer them or not!

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Let me know what you think – and also let me know if this little introduction to harmonising in four parts has been of any use to you!
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