Orchestration is the study or practice of writing music for an orchestra (or, more loosely, for any musical ensemble, such as a concert band) or of adapting music composed for another medium for an orchestra. Also called "instrumentation", orchestration is the assignment of different instruments to play the different parts (e.g., melody, bassline, etc.) of a musical work. For example, a work for solo piano could be adapted and orchestrated so that an orchestra could perform the piece, or a concert band piece could be orchestrated for a symphony orchestra.
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In classical music, composers have historically orchestrated their own music. Only gradually over the course of music history did orchestration come to be regarded as a separate compositional art and profession in itself. In modern classical music, composers almost invariably orchestrate their own work.
However, in musical theatre, film music and other commercial media, it is customary to use orchestrators and arrangers to one degree or another, since time constraints and/or the level of training of composers may preclude them orchestrating the music themselves.
The precise role of the orchestrator in film music is highly variable, and depends greatly on the needs and skill set of the particular composer.
In jazz big bands, the composer or songwriter writes the lead sheet, which contains the melody and the chords, and then one or more orchestrators or arrangers "flesh out" these basic musical ideas by creating parts for the saxophones, trumpets, trombones, and the rhythm section (bass, piano/jazz guitar/Hammond organ, drums).