Difference between In part and In parts

in part—partly; to some extent:

• The improvement was brought about in part by the Trade Union Movement.

Cf.: in good part—to a great extent; largely:

• These mechanisms are intensely studied and in good part understood.

in parts—

1. (of a book) published in installments:

• Once the craze was all for issuing volumes in parts; today part publication is almost unknown.

2. (of a melody) performed by particular voices or instruments:

• The old barcarole was sung in parts, at stem and stern of the same boat, by its own gondoliers.