Full Idea
There are four cases of possible forms of expression when a is made up of b: we say 'b is part of a', or 'b is a part of a', or 'b are a part of a', or 'b are parts of a'.
Gist of Idea
We say 'b is part of a', 'b is a part of a', 'b are a part of a', or 'b are parts of a'.
Source
Peter Simons (Parts [1987], 6.4)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.234
A Reaction
Personally I don't want to make much of these observations of normal English usage, but they are still interesting, and Simons offers a nice discussion of them.