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1. A comma follows introductory lead-ins coming before the main sentence:

a. adverb clause (usually):
Although the squirrel was just a baby, he still collected acorns for his family.
b. -ing word group:
Collecting acorns, the baby squirrel felt industrious.
c. prepositional phrase that is lengthy or could be misunderstood:
With much chattering, the squirrel dashed up the tree trunk.
d. infinitive phrase (an infinitive is to with a verb):
To climb the tree, the squirrel had to dig his claws into the bark.
e. words or phrases that could be misread:
Above, the branch swayed with the squirrel's romping.

2. Put a comma before a coordinating conjunction (and, but or, nor, for, so, yet) when it connects two independent clauses unless the clauses are very short:
The squirrel dropped the acorn, and a pedestrian kicked it into the street.

3. Put a semicolon between two independent clauses that do not have a coordinating conjunction between them:
The squirrel dropped the acorn; and a pedestrian kicked it into the street.

4. When a conjunctive adverb (however, consequently, thus, otherwise, in fact, furthermore, instead, likewise, moreover, then, still, also, nevertheless, for example) connects two independent clauses, place a semicolon before it and a comma after it:
The squirrel dropped the acorn; however, the pedestrian kicked it into the street.

5. Put a comma on each side of an interrupter within an independent clause. A conjunctive adverb interrupting one independent clause is also set off by commas:
The squirrel, I think, was upset with the pedestrian.
The squirrel determined, therefore, to pellet acorns at all passing pedestrians.

6. An -ing word group that comes after the word it modifies may be an interrupter:
The squirrel, chattering and flicking his tail, alerted the neighborhood.

7. Set off interrupters such as non-restrictive clauses or appositives with commas. Do not set off word groups needed to identify (restrict) a word:
The squirrel, which was one of the black variety, had a good supply of acorns. OR The squirrel which was on the highest branch is the one that made the most noise.

8. Do not place a comma between an independent clause and an adverb clause that follows it:
The rest of the squirrels ran and hid when the first squirrel called out the alarm.

9. Place a comma between the parts of a series of three or more words or phrases. The comma before the next-to-last item in a series may be omitted, especially in newspaper writing, but it must be there if the sentence might be misunderstood without it. It is never wrong to include the comma in that position:
The squirrel zoomed up the tree trunk, disappeared into its nest, and continued scolding passing pedestrians.

10. Place a comma between independent adjectives. If the adjectives can be reversed or the word and placed between them without changing the meaning, then the adjectives are independent and need commas:
The mischievous, clever squirrel pounced on the cat from behind the tree trunk.

11. Place a comma after an independent clause when an afterthought follows it:
The cat screeched and ran, coward that he was.

Unnecessary Commas

Do not place commas in these places:

1. Between a subject and verb unless an interrupter separates them:
INCORRECT: The squirrel on the lowest branch, calmly ate an acorn.
CORRECT: The squirrel, calmly eating an acorn, sat on the lowest branch.

2. Between a verb and its object:
INCORRECT: The critter proved, that squirrels have feelings, too.

3. Between two words or phrases joined by and or another coordinating conjunction:
INCORRECT: The squirrel jumped from the branch to the roof, and scampered behind the chimney. OR The squirrel behind the chimney, and the one under the tree had eaten their fill of acorns. OR The squirrels are either used to people being close by, or are more concerned about eating.

4. Around a needed word or phrase:
INCORRECT: The acorn, that he dropped, was the biggest I've ever seen.

5. After the last item in a series:
INCORRECT (third commas only): The tired, happy, plump, squirrel was done eating acorns for the day. OR He cleaned his face, rubbed his eyes, and settled into his nest, in the fork of the tree.