Idée fixe
Recurring main motif in Berlioz.
What it means
Idée fixe — “fixed idea” — is the designation that Hector Berlioz gave to a theme that runs through all the movements of his “Symphonie fantastique.” The theme embodies a notion that pursues the protagonist of the underlying programme incessantly, and therefore appears in every movement, always recognizable but in altered shape. The expression comes from the language of the time and designated a compulsive, recurring notion.
Musical use
The procedure combines two ideas: the unity of a multi-movement work through a common theme and the depiction of an extra-musical notion through a musical idea. The theme is not merely repeated but transformed — in rhythm, tempo, instrumentation, and character — so that it takes up the respective context without losing its identity. This transformation is the actual novelty compared with a mere return.
The principle stands in a series of related procedures of the nineteenth century. The idea of connecting multi-movement works through common thematic material was later developed further in various forms; in opera the idea of marking persons and notions through motifs led to the technique of the leitmotif.
Bear in mind that the term in the narrower sense is bound to this one work and in the wider sense is used for comparable procedures. In the narrower sense the term means the motif in Berlioz.
The idée fixe is to be distinguished from the identifying motif and leitmotif of opera, which are tied to characters or objects of a plot, and from the main theme, which need not carry an extra-musical meaning.
In the Symphonie fantastique the motif embodies the artist's beloved and appears in all five movements, each time adapted to the character of the movement.
Performance practice
Make the theme recognizable at every appearance, even when it appears transformed — it is the red thread of the work. Shape the transformation deliberately: it tells how the notion changes in the course. And read the programme to which the work refers; it explains the places where the theme appears.