Orchestral Parts File

a set of the orchestral parts of a musical work, composed or arranged for a small instrumental group. The parts file makes it possible for a work to be played by a varying number of performers, ranging from the minimal (“obligatory parts”) to the maximal (“desirable parts”). The obligatory parts of the file contain the reduction, a supplementary staff on which the principal melodic material of the other parts is written. In case there is no performer for these parts, they can be played by one of the performers of an obligatory part.