Bach's Invention No. 1 in C major began as one page in a keyboard notebook — the one he started in 1720 for his nine-year-old eldest son, Wilhelm Friedemann. Three years later he gathered those pieces into a fair copy and wrote on the title page exactly what they were for, a sentence still printed at the front of every edition. 1:10, with the full piano score on screen, note by note.
He wrote on the cover what the book was to teach
In 1720 Bach started the Clavier-Büchlein für Wilhelm Friedemann Bach for his eldest son, then nine years old. This piece first appears there, not yet called an invention but a praeambulum.
In 1723 he copied fifteen two-part and fifteen three-part pieces into a fair copy and wrote a title page describing the book as a sincere guide for lovers of the keyboard: to learn to play cleanly in two parts, and then to handle three; to learn to come up with good musical ideas and to develop them properly; and above all to acquire a singing manner of playing.
That sentence turns the book from a set of exercises into something else — what he is teaching is not the fingers but how to say a sentence clearly. Three hundred years later almost everyone who learns the piano still meets it.
Two hands saying the same sentence, one after the other
The right hand says something; two beats later the left says the same thing an octave lower. It is not accompaniment — it is the same sentence again.
After that the two keep trading: one speaks, the other answers, they move somewhere new and do it again. There is no melody-and-accompaniment here; the two hands play almost exactly the same number of notes.
And it never gets busy, because there are only ever two lines — at any moment there are exactly two things to follow.
One small shape, an eighth of a second long, carries the whole piece
The seven notes at the start are the seed of everything. Almost all of what follows is that shape moved: transposed up, turned upside down, stretched to twice the length.
For the first six bars the hands pass it back and forth. From bar 7 the left hand takes the running line while the right holds longer notes. Bars 15 to 18 thin both hands to eight notes a bar, the same shape stepping downward. At bar 19 the running line returns to the right hand, and in the last bar each hand holds a single long note.
Twenty-two bars, 466 sounding notes: 245 in the right hand, 221 in the left — less than a tenth apart, which is what "two equal voices" looks like as a number.
The hard part is keeping the hands from watching each other
Nothing here is fast. The difficulty is that each hand has to finish its own sentence — one running in sixteenths while the other holds still — and both are happening in one head.
Bars 7 to 12 and 15 to 18 are where they stick together most easily: the two shapes are so alike that one hand starts following the other's rhythm.
Around bar 10 the left hand climbs into the upper register; that passage is written in treble clef, so those notes are read on ledger lines on the lower stave — four at the most.
Fourteen more in the same book
There are fifteen two-part inventions (BWV 772–786) and fifteen three-part sinfonias; more will follow.
For more of one hand doing two jobs at once: the Canon in D solo advanced version, and Burgmüller's Arabesque.
