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

PUNCTUATION
Ensure that the power switch which is on the left side of the panel is
turned on.
These sentences present a conflict. In the first, the word which
implies that the clause which were heavily water-damaged is
nonrestrictive, meaning that all the prints were damaged-but the
absence of commas implies that it is restrictive, and that only
certain prints were damaged. In the second, the use of which
implies that there is only one switch and it happens to be on the
left side-but the absence of commas implies that of the various
switches, the one on the left is the one that should be turned on.
The result in both cases is ambiguity, which can be resolved only
if the text contains prior information that clarifies which possibility
is intended. In the absence of such information, it is not clear if
the writer erred in using which for that, or in omitting the commas.
Remember the general rule: Which takes a comma, that does not.
A final comment on the which/that rule: It is a North American
convention, not a universal one. British writers will happily use
which for restrictive clauses, and indeed in most cases the meaning
is clear. Disregarding the distinction is not a problem as long as
you know to avoid constructions that may be misconstrued.
SEPARATING ELEMENTS IN A SERIES
What's the difference between the following two sentences?
The only ones to show up were Monica and Bram, Sally, Bruce and
Penelope, Arthur and Ethel, and Humphrey.
The only ones to show up were Monica and Bram, Sally, Bruce and
Penelope, Arthur, and Ethel and Humphrey.
In case A, Arthur and Ethel came as a couple; Humphrey is odd
man out. In case B, the couple are Ethel and Humphrey, with Arthur
on his own. If there were no commas to show where the breaks
between elements occur, the sentence could be interpreted either
way.
Whenever you construct a sentence that contains a list of elements
of equal grammatical weight, you must use commas to separate
them. The elements can range from single words to phrases and
clauses.
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