The ritornello is composed of two sections.

• The first (measures 1 to 6) presents the thematic material that will serve as the basis for the entire movement. It begins in F major and from the second measure modulates to C major.
• The second section (measures 6 to 13), develops the motives of the first measures and immediately returns to F major.

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Let's see how Bach orchestrates the first phrase (measures 1 to 6). The upper voice is introduced by the first violin. The second violin and viola add a simple counterpoint to the first violin's phrase:

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The first violin is doubled by the piccolo violin and the cello is doubled by the continuo, violino grosso (contrabass) and bassoon:

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The first oboe seems to begin with a counterpoint, but immediately transitions to doubling the first violin:

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The oboes 2 and 3 add a counterpoint to the first oboe, sometimes in unison with the second violin and viola:

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The second horn starts doubling the first violin. The first horn imitates it in the second measure. The notes repeated in triplets will reappear throughout the movement:

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The second section of the ritornello (measures 6 to 13) develops motives from the first phrase (shown using colors). Note the use of inversion (I) and retrograde motion (R) in developing the sixteenth notes:

Reduction of measures 6 to 13:

• The red motive occasionally uses joint degrees instead of arpeggios.
• The blue motive appears rhythmically varied. Its first two notes are now sixteenth notes instead of eighth notes.

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Bach orchestrates the three voices we have just discussed in the following manner (score in concert pitch):

• The two upper voices are sometimes divided and sometimes doubled between the first violin, piccolo violin and first oboe.
• The bass is divided/shared between cello, bassoon, double bass and continuo.
• The second violin and viola and oboes 2 and 3 provide counterpoint to the upper voices.

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We can subdivide this first Episode into three sections. The first section (measures 13 to 18) can be reduced to four voices. We start in F major and modulate to C major. We have pointed out the motives of the first phrase. Notice the green motive in its original form and transformed by inversion (measure 14, third voice):

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Here is how these four voices are orchestrated:

• The second horn makes a counterpoint to the first horn.
• The oboes 2 and 3 play counterpoint to the first oboe.
• The second violin and viola sometimes double the first violin and sometimes play counterpoint.

Here is the orchestrated passage:

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Measures 18 to 24 take us to B flat major. The passage uses two voices, but the upper voice is divided among the violins, oboes, and horns:

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Here is the orchestrated passage:

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The passage from measures 24 to 27 takes us through a cycle of fifths from B-flat major to D minor. The upper voice passes from the horns to the oboes and finally to the strings. We see the red and blue motives:

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Here is the orchestrated passage:

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The second ritornello is an abbreviated version in the key of D minor:

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Here is the orchestrated passage:

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This section begins with two sixteenth-note motives of the first phrase:

A series of dominant sevenths (D - G - C - F) and then a series of descending chords (F - Eb - D - C) leads us to F major. We see the chords in the descending scales and arpeggios (measure 36 onwards) in eighth notes:

The series of descending seventh chords by degrees in measures 36 to 38 may surprise. They are actually part of the cycle of fifths compressed F - (Bb - Eb) - (A - D) - (G - C) - F:

When reaching F major in measure 38, the F chord functions as the IV degree of C major. The following measures affirm the new key:

Here is the orchestrated passage:

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The third ritornello is presented in abbreviated form. It begins in C major and modulates to A minor:

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Here is the orchestrated passage:

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This Episode takes us from A minor to G minor and then from G minor to F major using two cycles of fifths:

Here is the orchestrated passage:

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The fourth ritornello has only 6 measures. It begins in F major and modulates to C major:

Here is the orchestrated passage:

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This episode begins with a cycle of fifths using dominant sevenths in measures 63 to 65. We find the descending scales and the semiquaver motives of Episode 2 again:

In measure 66 a dominant F chord leads to B flat major:

The B-flat major chord is used as the IV degree of F major. An ascending march brings us to the climactic point in measures 69 to 70, where an auxiliary diminished seventh chord of the V degree leads to the cadence to F major:

Here is the orchestrated passage:

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