Here's why:
* Subject and Answer: A fugue begins with a short melodic phrase called the subject. This subject is then imitated by another voice, creating the answer. The answer is typically a transposition of the subject, often in a different key.
* Episodes: Between the entrances of the subject and answer, there are sections called episodes which connect the different voices and provide harmonic and melodic variety.
* Polyphonic Form: Fugues are written in a polyphonic style, meaning that multiple independent melodic lines are woven together simultaneously. This creates complex textures and harmonies.
* Highly Developed: Fugues are known for their intricate structure and often involve complex counterpoint, modulation, and development of the musical material.
Other examples of instrumental genres using polyphony:
* Sonata: A genre for solo instrument or a small ensemble, usually in multiple movements with different tempos and forms. While polyphony can be present, it's not the defining characteristic like in a fugue.
* Symphony: A large-scale work for orchestra, typically in four movements. Polyphony is often used in specific sections of a symphony but not the primary defining feature.
* Concerto: A work for solo instrument with orchestra accompaniment. While polyphonic elements can be present, the focus is often on the interplay between soloist and orchestra.
While other genres can employ some polyphonic elements, the fugue is uniquely defined by its use of a subject, answer, episodes, and highly developed polyphonic structure.