Why Is a Fugue More Than Several Melodies at Once?

Several independent melodies sounding together establish a polyphonic texture, but that alone does not make a fugue. The centre of a fugue is a recognisable subject entering several voices while retaining its identity through changing relationships. One voice normally states the subject, another responds with an answer, and further voices join to complete the exposition. What matters is not that everyone speaks at once, but that each arrival addresses the same musical question.

After the exposition, the subject may disappear while the music moves through connecting or developmental episodes. It returns in different keys, voices and pitch positions. A composer may also transform it through inversion, augmentation, diminution or stretto. The listener should follow when the subject enters, where it comes from, and how the other voices make room for, resist or complete it. The fugues of Bach’s Well-Tempered Clavier are particularly good for cultivating this kind of hearing because each one gives the relationship between subject and voices a different character.

For a pianist, the challenge is not merely finger independence. If every voice is equally prominent, the structure becomes a crowd of lines; if only the top line is projected, the fugue becomes melody with accompaniment. A better approach is to locate each subject entry and then shape the current hierarchy, making the subject audible without brutally suppressing the other voices. The beauty of fugue is not mathematical neatness. It is the ability of one piece of material to remain coherent through difference, modulation and reorganisation. Fugue is music about memory: we hear what the subject has become because we recognise what it was.

https://www.britannica.com/art/fugue
https://imslp.org/wiki/Well-Tempered_Clavier%2C_Book_1%2C_BWV_846-869_%28Bach%2C_Johann_Sebastian%29