It is a matter of opinion whether there is a difference between orchestration, arranging and instrumentation and the terms are often used as synonyms. However, if the terms are judged different their difference are generally thought to be the following: Instrumentation deals with the techniques of writing music for a specific instrument, including the limitations of the instrument, playing techniques and idiomatic handling of the instrument. Orchestration includes, in addition to instrumentation, the handling of groups of instruments and their balance and interaction. As such, instrumentation and orchestration can be seen as more handicraft skills than addition of musical content. However, in the hands of a skilled orchestrator, matters such as whether an oboe or a clarinet plays a particular melody can be turned into key elements of a piece.
Arranging, in contrast, is the setting of music or melody for other instruments than it was originally written. In this process, arranging can include addition of musical content such as, creation of secondary melody lines or new musical contexts giving the melody new depth.
Although most composers do their own orchestration, they sometimes use an orchestrator to do this job for them.
An orchestrator will usually be presented with a piece in short score (that is, written on around three or four musical staves) or else the piece will be written as if it were to be played on a piano. They will then have to decide which musical instruments are to play which notes. Percussion effects may be marked in the short score, or may be left to the discretion of the orchestrator. The exact amount of work an orchestrator has to do can vary, but in almost all cases he is called upon only when all the other work on a piece has been done.
The job of a dedicated orchestrator is mostly seen as skilled work, as opposed to the "inspired creativity" of a composer--though many composers who are known for work in their own right have worked as orchestrators to earn extra money. The influential classical composer Anton Webern for example, worked orchestrating operettas. Quite well known composers of musicals and film music often do not do their own orchestration, although the person who does do this work is often overlooked.
A well known example of a piece that orchestrators worked on is the musical West Side Story (later turned into a film). Although the music was written by Leonard Bernstein who usually receives the sole composer credit, much of the orchestration was carried out by Sid Ramin and Irwin Kostal.
Difference between orchestration, arrangement and instrumentation
Dedicated orchestrators
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