What Bach’s Prelude BWV 998 Can Teach Us About Composition
Bach can be deceptively difficult to understand.
We hear a piece such as the Prelude, BWV 998 and immediately sense that the music is coherent. It has direction. It sounds inevitable. Yet when we put the score in front of us and try to explain why it works, the structure can be surprisingly difficult to see.
Where is the A section? Where is the B section? What are the chords? Why does the music sound unified when it seems to be constantly moving somewhere new?
These questions came up recently in a lesson while analysing Bach's Prelude, BWV 998. The student described the piece as seeming almost "without structure." That observation actually provides a very useful starting point, because the Prelude shows us that musical structure doesn't always mean obvious sections such as binary, ternary or ABA form.
Sometimes structure comes from something much simpler:
An idea that keeps returning.
The Prelude as "Notated Improvisation"
Many preludes can be thought of as through-composed. Rather than being organised into clearly repeated sections, the music continually unfolds.
This connects with the historical relationship between preluding and improvisation. A prelude could function as something spontaneous: an introduction, an exploration of a key, or a musical thought that unfolds freely.
In that sense, a written prelude can sometimes resemble notated improvisation.
When improvising, you don't necessarily think:
"Now I will compose an eight-bar A section, repeat it, and then begin my B section."
Instead, you might begin with an idea and explore where it can go.
That could be:
a melodic motive
a rhythmic idea
an arpeggio pattern
a harmonic progression
a texture
The repeated idea becomes the thread holding the music together.
We see this very clearly in preludes built almost entirely around a single arpeggio pattern. The harmony changes continually, but the recurring pattern creates unity. The music doesn't need an obvious ABA structure because repetition is already doing the structural work.
Bach's BWV 998 Prelude does something similar, but in a more sophisticated way.
Follow the Theme
One of the most important things to identify in BWV 998 is the melodic idea heard at the beginning.
Rather than disappearing after its first appearance, this idea returns several times in different keys.
In the lesson, we traced several important appearances:
Beginning — D major
The theme begins in the tonic, establishing the home key.
Measure 6 — A major
The theme returns in the dominant.
Measure 14 — B minor
We hear it in the relative minor.
Measure 25 — G major
It appears in the subdominant.
Towards the end — D major
The music eventually returns home to the tonic.
Suddenly, something that initially appears to lack structure begins to make much more sense.
The structure isn't primarily:
A → B → A
Instead, we can hear it as the journey of a musical idea:
Tonic → Dominant → Relative Minor → Subdominant → Tonic
The melody gives us something recognisable.
The changing keys give us movement.
And the return to D major gives us a sense of homecoming.
Don't Just Analyse Chords — Analyse Keys
This also explains why analysing Bach can sometimes feel frustrating.
Students are often taught to analyse harmony by identifying individual chords:
I – IV – V – I, for example.
But BWV 998 isn't always presenting us with convenient block chords or obvious arpeggiated harmonies. Much of the writing consists of an elaborate melodic line, sometimes combined with a bass, with the harmony implied rather than explicitly laid out.
So instead of constantly asking:
"What chord is this?"
a better question is often:
"What key are we in right now?"
This change in perspective can make Bach considerably easier to understand.
Bach also doesn't necessarily remain in one key for very long. In this Prelude, the tonal landscape can change within only a handful of measures. That makes a chord-by-chord analysis difficult if we expect one key to remain stable for a long period.
Try stepping back.
Find the larger tonal destinations first.
Then examine how Bach travels between them.
A Composition Exercise Hidden Inside Bach
This analysis immediately gives us a practical composition exercise.
Write a short melody in a major key.
Then try presenting that same melody in:
the tonic
the dominant
the relative minor
the subdominant
the tonic again
You don't have to copy Bach's Prelude note for note. You're borrowing the principle behind it.
Simply transposing one melodic idea into these related keys can generate a surprising amount of material.
This is one of the great benefits of analysing master composers: analysis stops being merely theoretical.
Instead of asking only:
"What did Bach do?"
we begin asking:
"What can I create using the same principle?"
Unity and Variety
This leads to one of the fundamental problems every composer has to solve: balancing unity and variety.
Too much unity and the music becomes predictable.
Imagine repeating the same melody in D major again and again. Eventually the listener knows exactly what is coming.
But too much variety creates the opposite problem.
If every few measures contain a completely new theme, new texture and new idea, the listener may struggle to understand what the piece is about.
Bach finds the balance beautifully.
The unity comes from the recurring melodic material.
The variety comes from hearing that material in different keys and contexts.
This is an extraordinarily useful principle for composers at any level:
Don't assume that developing a piece means constantly inventing new material.
Often the better question is:
What else can I do with the material I already have?
Transpose it.
Change the bass.
Change the harmony.
Alter the register.
Decorate the melody.
Change the texture.
Move it into minor.
Return to it later.
Composition is often less about producing endless ideas and more about learning how to develop a good one.
Inspiration Needs Craft
This brings us to another important distinction: the relationship between inspiration and craftsmanship.
A melody may arrive spontaneously. That's wonderful.
But then what?
Where does it go?
How do you turn eight seconds of inspiration into three minutes of coherent music?
This is where craft becomes essential.
During the lesson we discussed the idea attributed to Brahms that:
"Inspiration without craftsmanship is a mere reed shaken in the wind."
The important principle is that inspiration alone doesn't finish a composition. A beautiful musical idea needs techniques that allow the composer to develop, structure and complete it.
Bach had both.
There is musical imagination, but there is also an extraordinary command of melody, harmony, counterpoint, form and musical structure.
Composition Is Like Language
One helpful way to think about this is through language.
When we speak our native language, we don't consciously analyse every noun, verb and grammatical construction before opening our mouths. Years of experience have internalised those things.
Something similar happens with music.
A highly trained composer has absorbed patterns of melody, harmony, counterpoint and form to such an extent that they become part of a musical vocabulary.
This also gives us a useful analogy between improvisation and composition.
Improvisation is like speaking.
Composition is like writing.
Speaking allows ideas to emerge immediately.
Writing allows us to stop, reconsider, edit, delete, rearrange and refine.
In the same way, improvisation can provide the raw musical thought, while composition gives us the opportunity to craft that thought into something more deliberate.
The two don't have to be enemies.
Improvisation can be the genesis of composition.
Model the Masters
So how can we actually learn to do this ourselves?
One of my favourite approaches is what I call model composing.
Rather than staring at a blank page and trying to "be original," take a piece by a great composer and use its underlying process as a model.
For BWV 998, you might begin with:
Bass note → melody → bass note → melody
Then gradually allow the voices to interact.
Create your own melody.
Make it more ornate.
Add passing notes.
Experiment with a bass.
Move the idea to the dominant.
Then ask: What does Bach do next?
You aren't copying Bach's notes. You're learning to think through the compositional problems that Bach solved.
In the lesson, this became a practical exercise: create a melody, ornament it, add a bass voice, then emulate the way Bach moves the material into another tonal area.
This is one of the fastest ways I know to turn analysis into genuine musical understanding.
Analysis Should Change the Way You Play
Finally, this kind of analysis isn't only for composers.
It should affect performance.
Once you recognise the return of the opening theme in A major, B minor, G major and finally D major, those moments are no longer simply strings of notes.
They have meaning.
You can make the listener hear:
Here is the idea.
Now it has travelled somewhere new.
Now its character has changed.
And now, finally, we've returned home.
That knowledge can inform dynamics, phrasing, tone colour, pacing and rubato.
This is why theory and performance should never be completely separated.
Analysis helps us discover what the music is doing.
Composition helps us understand why those techniques work.
Performance gives those discoveries a voice.
And studying Bach gives us the opportunity to practise all three at once.
Try This Yourself
Choose a short piece you're currently learning and resist the temptation to begin with Roman numerals.
Instead, ask:
What is the main musical idea?
Find every place it returns.
Then ask:
What key is it in each time?
Look at what remains the same and what changes.
Finally, take the underlying principle and write four or eight measures of your own music using it.
That's when analysis becomes more than an academic exercise.
It becomes a creative tool.