How did Mozart use counterpoint while keeping his music organized by harmony and key?

In the late Classical period, Wolfgang Amadeus Mozart integrated counterpoint into the sonata form by ensuring that his combined melodic lines remained subservient to the underlying harmony and tonal design of the work.1

Mozart achieved this balance through several key methods:

• Unified Phrase Structure: While he utilized independent melodic lines, each voice was governed by an underlying phrase structure. This ensured that the combined parts formed unified musical phrases, satisfying the Classical demand for symmetrical lengths and clear-cut cadences that mark the sections of a sonata.1
• Harmonic Subservience: Even in complex contrapuntal passages—such as the simultaneous statement of five themes in his Jupiter Symphony—the counterpoint was kept entirely subordinate to the symphony's tonal plan.1
• Thematic Integration: In the development section of sonata form, Mozart often brought themes or fragments into new contrapuntal relations, allowing them to be played against each other as countermelodies while still navigating the movement's tonal flux.2
• Contextual Nuance: Mozart sometimes used counterpoint to create ambiguity, such as in his Symphony No. 35 (Haffner), where a second theme is played as a counterpoint to the first, effectively creating a new perspective on the tonic key rather than simply moving to the dominant.3