Commas are one of the most misused (and misunderstood) punctuation marks. Not only are comma rules complicated, they also have a great deal of stylistic variation.
1. How they are used has changed over time.
I love complicated sentences with many phrases off-set by commas and semicolons because many of my favorite literary works are from 19th century England. Some people prefer the shorter, more concise sentences seen in US writing since the mid-20th century. We often develop our writing styles and grammatical preferences from the writing we love, which can also cause a lot of consternation between people of different stylistic origins.
2. There is a lot of variation in regional, professional, and personal styles.
Comma usage changes depending on where in the world the writer is from (or where they learned English) and if the work is a translation from a different language. Professional style guides, such as American Psychological Association (APA) style or AP (Associate Press) style, have different rules for how to use commas. Most professional organizations and publishing companies have formal style guides that describe how commas are used.
3. They are a kind of grammatical multi-tool.
At their core, commas are used to separate closely connected ideas and clauses. Some writers will use them to reflect a verbal pause, especially in dialogue or when the writing style is intended to reflect a spoken cadence.
Here is a general outline of common comma uses and rules. I will have some resources at the end to reference other grammar rules and more complex or unusual comma rules. You should always check your organization’s style guide if you are writing professionally.
Dependent clauses cannot be made their own sentence without changing the structure. They usually start with although, whenever, if, since, when, and because.
1. Use a comma if the sentence starts with a dependent clause Example: When he washed dishes, he wore gloves and an apron.
2. Do not need a comma if the dependent clause comes after the main clause Example: He wore gloves and an apron when he washed dishes.
3. Commas are optional if (a) the dependent clause starts with a preposition or (b) the dependent clause is short (a)Example: In the cupboard are three boxes of cookies. (b)Example: When lost she opens her GPS.
4. Always use commas if it will clarify the sentence’s meaning. Example: Last Tuesday, courses were cancelled.
Independent clauses can stand on their own if they are removed from the sentence.
1. You usually do not need a comma if the subject is not in front of the second verb, but you should use one if it make the sentence easier to understand. Example: She ate cake and drank milk.
2. Use a comma at the end of the first clause if you are linking two independent clauses with a connector, such as and, but, or. Example: I want to go to the library, but it is storming.
3. A comma splice is when a writer uses a comma where a period should be. Usually they are connecting two independent clauses without showing how they are related. Example: I fed the pets, I went for a run.
>> Add a preposition to the second clause. Example: I fed the pets, then I went for a run
>> Make them two separate sentences. Example: I fed the pets. I went for a run.
>> Add a preposition before the first clause. Example: After I fed the pets, I went for a run.
Commas are used to separate items in a list of three or more. There is some considerable debate over whether there should be a comma placed after the second to last item, which is called the Oxford comma or serial comma. Example: Shelby wore a dress, boots and a scarf. Example: Shelby wore a dress, boots, and a scarf.
1. If you are writing in a professional capacity, check your style guide. Most organizations will have a preference on the serial comma.
2. If you are writing without a style guide, your primary goal should be clarity. I prefer the serial comma in my personal writing, but there are many who find it unnecessary.
You should use commas to separate interchangeable a