The Art of Simplicity: J.S. Bach’s Two-Part Invention No. 13 in A Minor, BWV 784 as a Polyphonic Masterpiece

1. The ‘Second Rise’ of Baroque Music

   
   

In 1802, German music historian Johann Nikolaus Forkel (1949-1818) published the first scholarly biography of Johann Sebastian Bach, Über Johann Sebastian Bachs Leben, Kunst und Kunstwerke (On Johann Sebastian Bach’s Life, Art, and Works). This pivotal work marked a turning point in how Bach was perceived, rescuing him from near-total posthumous obscurity. The emergence of musicology as a branch of modern humanities, particularly during the Romantic era, intensified interest in music of earlier periods. As a result, composers and audiences alike began rediscovering, publishing, and performing works from the past—placing Bach at the center of this revived canon. Unlike during his lifetime or the Classical era, Bach came to be regarded in the Romantic period as a towering musical figure, arguably more esteemed than any of his predecessors. Indeed, following Mendelssohn’s revival of Bach’s St. Matthew Passion, virtually no composer of note neglected to study Bach’s works. This renewed attention also sparked broader interest in other composers whom Bach had studied or arranged—especially Vivaldi—leading to a broader revival of Baroque music that gained momentum in the late 19th century.

2.  Keyboard Music and Bach

Have you heard the phrase, “If all other sheet music were lost and only Bach’s Well-Tempered Clavier remained, Western music history could begin again from the Baroque”? This oft-quoted remark speaks to The Well-Tempered Clavier’s significance as a comprehensive collection of Bach’s contrapuntal techniques and harmonic language. In addition to this monumental work, Bach’s music—ranging from the widely beloved Inventions, the Brandenburg Concerto, to the Solo Cello Suites—continues to be performed and revered. But why do musicologists today often regard Bach’s works as perfect? One of the most crucial criteria in scholarly and critical evaluation is whether a work introduces a genuinely original approach. In the Renaissance era before the Baroque, instruments were tuned using various systems of just intonation, often making ensemble playing problematic due to incompatibilities between keys. The concept of “well-temperament” emerged as a solution. Although Bach did not invent well-tempered tuning, he thoroughly understood it and explored its full potential by composing preludes and fugues in all 24 major and minor keys, thereby demonstrating its expressive scope. Bach’s Well-Tempered Clavier, the first complete exploration of this kind, not only expanded the possibilities of keyboard music but also helped to popularize the well-tempered system. Today, it holds a central place in the long history of tonal and keyboard music.

3.    Polyphony and Bach’s Two Part Invention: Is Bach’s Invention no.13 something innovative and perfect?

First, before we discuss on Bach’s Invention no. 13, I would like to introduce several musical terms to you. To analyze Bach’s music, we need to know about the concept of texture in music and the history of polyphony. Texture means the design and the figure of the layer of voices when it is used in music. Polyphony simply refers to the music which has multiple voices. Music, which began as monophony (we refer to music as monophonic when it has only one voice without any accompaniment), evolved into polyphony by the late Renaissance, following the development of medieval church music. Early polyphony which emerged around the 9th century, took the form of first species (1:1) counterpoint, in which one consonant interval is added respectively above or below one single note of a given melody (the tenor). Over time, this practice became increasingly complex, with longer melodic lines to one note on tenor, ornamentations, and accidentals added to the tenor voice. From this point, we shall refer to this kind of music as polyphony. However, we need to distinguish between polyphony and polyphonic texture. Polyphonic texture refers to musical texture that combines two or more independent melodic voices played or sung at the same time. Thus, to be called polyphonic, each line needs to have its own distinct melody, but they are designed to harmonize and interact with each other.

Now, are you ready to see  Bach’s Invention No.13 from the perspective of polyphony and polyphonic texture that we have discussed? Though every single work from Bach’s keyboard works represents the pinnacle of polyphony, we will focus on the No.13 today.

< J.S. Bach: Invention No. 13 in A Minor, BWV 784, performed by Andras Schiff>

Can you see two independent voices on the sheet music above? Simply, the voices are divided into two parts: the right hand and the left hand. Thus, this piece is classified as a two-part invention. The opening theme begins in the right hand with an arpeggiation of an A minor chord in sixteenth notes, which is subsequently echoed an octave lower in the second voice, played by the left hand. In contrast, right after the sixteen notes, the second material comes with the chordal leaps presented in eighth notes. Chordal leaps occur when melodic leaps are confined within a single chord structure, creating harmonic motion. In the first four measures, these two elements (arpeggiation and chordal leaps) turn to the form of a sequence in measures 3 and 4. Sequence refers to the repetition of a musical idea (motive or phrase) at a different pitch level, typically moving up or down stepwise.  In measure 5, the two elements (arpeggiation and chordal leaps), which were previously exchanged between the voices in half-measure units, is now compressed into one-beat intervals, initiating a more rapid and intensified form of imitation. This section resembles a fast canon or round, a technique referred to as stretto.

The interplay between the right and left hands is never merely decorative or functional; rather, it models true polyphonic dialogue—each voice retaining its own identity while participating in a cohesive musical argument. A simple motive introduced at the outset undergoes continuous development through sequences, inversions, and various imitative figures between the two voices. The compositional technique of developing music from minimal material was widely practiced during the Baroque period. However, Bach did not merely employ this technique as convention; in his Inventions, he elevated it into something musically compelling and aesthetically persuasive. To be sure, one cannot fully grasp Bach’s artistic significance without considering his more complex and expansive works, such as his fugues or French suites. Yet in the Two Part Inventio—the subject of today’s exploration—Bach demonstrates the pinnacle of polyphonic writing using only two voices!

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