It can be easy to think, sometimes, "who cares about commas? Heck, there's no real rules anyway. I see them used in wild ways all over the web. Therefore, I’ll go with whatever feels comfortable to me."
There are two challenges with this approach.
The first challenge is that comma rules exist for a reason. Their purpose is to clarify the meaning of the sentence. If commas are used improperly the sentence can be misinterpreted. You, the author, might not even realize it because you know what the sentence SHOULD mean. However, the reader is in the dark. They might get an entirely different meaning from the sentence based on the lack of or addition of commas.
Here's an example.
I ate the candy apple and popcorn.
I ate the candy, apple, and popcorn.
In one case I ate an item called a "candy apple". In the other I ate a handful of candy and also a fruit. The commas are necessary in order to separate distinct items in the list.
Yes, you might not care about comma usage. However, many of your readers do. We all have different tolerance levels for poorly written content. Imagine if you went to a well-respected website and found this sentence:
LOLZ can u believe sum people thik hummbirds are from Florida? Nope, they wintre in Mexiiko and travl up each year in what iz called "Migrasion".
You might hit the BACK button and immediately form a low opinion of that website because you feel the writing is such low quality. Where you have that instant reaction to poor spelling, others have that reaction to poor grammar. There's little "up side" to writing incorrectly - but there's a great deal of "down side" to it.
So here are some rules to remember when using commas.
COMMA IN A SERIES
This is the most common use of a comma. It's when you have a series of items, and you use a comma to separate those items. So here are some examples.
My wedding dress featured lace, beads, and pearl buttons.
The hiker carried a canteen, a compass, and a GPS.
My kitten is grey, striped, and cute.
The final comma in the set is considered by some to be optional. That is, some people would write:
My kitten is grey, striped and cute.
Using the final comma is called having an "Oxford Comma". It's generally recommended to use that Oxford Comma because it adds to clarity. That is, it never hurts to include it and in many sentences it can greatly help.
Here's an example that shows why the comma is important. The comma helps show which word or words belong together.
Ivory the parakeet is elderly, light blue, and friendly.
“Light blue” is a color. It’s not that Ivory is light and also blue. It’s that she’s a color which is “light blue”.
COMMA BETWEEN DESCRIPTIVE ADJECTIVES
Again, this is a use most people understand and use properly. If you have several adjectives describing something you use commas between them to separate them out.
I walked up to the large, stately, red brick house.
The birch tree had peeling, crisp, pale white bark.
The "red brick" and "pale white" are phrases that convey one sense. That is, the bark isn't pale and also white. It's a color which is pale white.
COMMA AFTER INTRODUCTORY WORD / PHRASE
If a word or phrase starts a sentence as an introduction, it should be set off by a comma. Here are a few examples.
Yes, I am inviting you to the wedding.
No, you cannot bring the machine gun to the wedding.
Therefore, you need to bring it back to the armory.
However, you can bring a ballpoint pen if you really need to.
Otherwise, you will never learn that the pen is mightier than the sword.
By leaving now, we will make sure we get there on time.
The above comma uses are ones that most people have a handle on. They "make sense". Now we head into the commas that can sometimes trip people up.
COMMA WITH NONRESTRICTIVE APPOSITIVE
An appositive is a description which comes after a noun. A Nonrestrictive appositive is a description which adds additional information that isn't necessary for the sentence to work. Examples might help here :).
Lisa's first murder mystery novel, Aspen Allegations, won a gold IPPY award.
The hummingbird, green with a ruby throat, flitted at the window.
In both cases the middle part between the commas - the "Aspen Allegations" and "green with a ruby throat" - could be removed from the sentence and the sentence still makes sense. So that is the part that needs commas.
In comparison, if I said:
Have you read my book Aspen Allegations?
There is no comma needed there because the "Aspen Allegations" part is necessary. Without that, you wouldn't know which of my books I was asking you about.
COMMA WITH PARENTHETIC EXPRESSION
This is a quite similar case. Again, we're talking about a phrase which is separate from the primary thrust of the sentence.
Aspen Allegations, which is set in Massachusetts, follows the explorations of a suspicious death.
The medieval heroine, who had an abusive father, sets out to join a nunnery.
So in both of those cases, with nonrestrictive appositives and parenthetic expressions, the key is to try pulling out the phrase and see if the sentence still can make sense. If so, you want to use commas to set that phrase apart.
Now we come to an area which can be quite confusing.
COMMAS AND INDEPENDENT CLAUSES
An independent clause has a noun and verb in it. There's a "something" and there's "what it is doing". This clause is in essence a sentence. In comparison, a dependent clause isn't a complete thought. It is a piece of a sentence. If two independent clauses are joined by an AND, BUT, or OR there should also be a comma to clarify the situation.
I went to New York City to accept my award, and I had some of the best pizza in the United States.
In the above sentence, if we chopped out the comma and AND we end up with two quite fine sentences:
I went to New York City to accept my award.
I had some of the best pizza in the United States.
So the test here is to pull the AND, BUT, or OR out of the sentence. Can the remaining halves stand alone as full, meaningful sentences? If so, you want to have a comma in there.
The one exception to this, and this is where "good judgment" comes into play, is if the two sentences are REALLY SHORT. So for example:
My speech was short and the crowd applauded.
Yes, technically we do have two complete sentences here:
My speech was short.
The crowd applauded.
Still, the sentence is only eight words long. There’s no need for a comma.
BITS AND PIECES
Then there are all the random little places that you use commas. You use them when listing a city and state / country:
I loved going to the ice hotel in Quebec, Canada.
You use commas when talking directly to a person.
Please remember, Lisa, to always wash your hands before eating sushi.
You need commas when quoting somebody.
"I want to be helpful," Jenny said, "so please let me know what I can do."
You need commas in a full date involving a month, date, and year.
Mary was born on April 1, 2011.
You also need a comma in numbers that are over 999.
Michelle won $15,253 from the lottery.
Feel free to let me know if I mis-used any commas in my write-up :).
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