Grammatically Correct: The writer's essential guide to punctuation ...

PUNCTUATION
The panel consisted of three professors, two historians and an economist.
How many people were on the panel: three or six? Being a professor
is not mutually exclusive with being a historian or an economist.
If the intention is to describe six distinct individuals, a serial comma
would clarify this: three professors, two historians, and an economist.
If the latter two occupations are the professors' specialties,
this part of the sentence should appear as three professors: two
historians and an economist. (For a discussion of this role of the
colon, turn to page 97.)
Similarly,
Before beginning assembly, ensure that you have all the following parts
and that you have paired those that go together: C-460, M-1 00 and R-
560, G-1 00, R-1 00 and T-340 and T-640.
Which is the pair here: R-100 and T-340, or T-340 and T-640? The
absence of a serial comma makes it unclear which parts constitute a
unit. Inclusion of the comma would clarify whether the writer means
R-1 00 and T-340, and T-640 or R-1 00, and T-340 and T-640.
Finally, sometimes the length or complexity of the elements in a
series calls for the use of the serial comma. For example:
Teachers need hours outside of class time to plan new curricula, grade
tests and papers, meet with parents, consult with specialists about
students with academic or behavioral problems, and work one-on-one
with children who have special needs.
The consultants independently ranked each survey item on its relevancy
and clarity using a 1 0-point scale, completed open-ended questions
pertaining to the appropriateness of the items, and offered suggestions
for the elimination or addition of items.
If the serial comma were omitted in these sentences, readers
might have to strain to see where one element ends and the next
begins.
In summary: If it is your choice not to use the serial comma,
that's fine, but let clarity override consistency if an exception seems
necessary.
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