The instrument which was used to help musicians know when to change pieces while accompanying early films in movie theaters are:
A cue sheet is a sheet which contains information about the things that are needed for a musician to know when a particular sound is needed and when he should play it.
As a result of this, this helped them to know the part of a classical work to play in order to rhyme or match the scene that was currently on display
Therefore, the correct answer s cue sheets
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