Did they come up with a melody and think ‘this will sound great on a violin’? Or did they have a whole orchestra at their disposal when they composed?
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They did not have an orchestra at their disposal to get quick feedback on how it would have sounded. They would have had to build that skill over many years of attending concerts and studying other musical works.
Also, they most likely built up to composing orchestral works, starting with instrumental solos and small chamber works like string quartets and piano trios.
Also also, there were strong musical conventions to follow during the common practice period, so that provided a lot of structure around which to organize a composition.
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