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Punctuation
1. Commas
Use a comma before and, but, so, yet, or, and nor when those words are used to join two main clauses.
EXAMPLES:
I think that Doré’s illustrations of Dante’s Divine Comedy are excellent
-
, but my favorite
drawing is “Don Quixote in His Library.”
Practically all nitrates are crystalline and readily soluble
-
, and they are characterized by marked
decrepitation when heated on charcoals by a blowpipe.
The general rule stated above should be qualified in two respects. First, when the two clauses joined by
the conjunction are very short, the comma is optional. In the following example, both sentences are correct.

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EXAMPLE:
The door was ajar and the house had been ransacked.
The door was ajar
-
, and the house had been ransacked.
Second, if either clause itself contains commas, you may need to use a semicolon before the con-
junction for clarity.
EXAMPLE:
Because many diseases and insects cause serious damage to crops, special national legislation
has been passed to provide for the quarantine of imported plants
-
; and under provisions of
various acts, inspectors are placed at ports of entry to prevent smugglers from bringing in
plants that might be dangerous.
Given the length of the two clauses and the fact that each clause contains a comma, you should use
a semicolon following plants, rather than a comma.
Use commas to separate the elements of a series.
EXAMPLES:
A full train crew consists of a motorman
-
, a brakeman
-
, a conductor
-
, and two ticket takers.
The procedure requires that you open the outer cover plate
-
, remove the thermostat
-
, replace the
broken switch
-
, and then replace the thermostat.
Use a comma to separate a subordinate clause at the beginning of a sentence from the main clause.
EXAMPLES:
After Peter finished painting the bird feeder
-
, he and Jack hung it from a limb of the oak tree.
When Pat explained to his mother that ten was the highest mark given on the entrance test
-
, she
breathed a sigh of relief.
If the subordinate clause follows the main clause, you do not need to set it off with a comma.