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Series index: English Punctuation Overview.

Commas

Commas separate words, phrases and clauses to clarify meaning. They often indicate pauses in speech, but not always. This Guide to Using Commas covers the basics, and this guide to The Comma refers to AP style guidelines. Here’s a more detailed look at comma usage:

To join two independent clauses - “I spent all day cooking this dinner, and the family ate it in three seconds.” Always use the comma before the coordinating conjunction (like and), but only use a comma if both clauses are independent. If the second clause doesn’t have its own subject, don’t use a comma (as in “I made dinner and set the table”). See The Joining Comma for more. Note: many find it acceptable to leave out the comma if both independent clauses are short (”I made dinner and they ate it”).

To show omitted words - “I like white chocolate; Bob, dark chocolate.” See The Gapping Comma. This applies to more specific uses as well (see also The Listing Comma), where the comma could be replaced with a word like and or or:

• In a series of three or more - “I like white chocolate, milk chocolate, and dark chocolate.” The comma stands for an omitted and or or. Many writers prefer not to use a comma before the conjunction: “I like white chocolate, milk chocolate and dark chocolate.” This optional comma, often called the serial comma or Oxford comma, has inspired lots of debate. See Wikipedia’s entry on the Serial Comma for more.

• With two coordinate adjectives - “She loves the cute, fuzzy kittens.” When you can’t insert the word and between the adjectives, don’t use a comma (”She is a nice old lady”, not “She is a nice and old lady”). See Commas with Paired Adjectives for more on the difference between coordinate and cumulative adjectives.

• With contrasting statements - “I like apples, not oranges.” I remember this rule by thinking of the comma as a replacement for and, as in “I like apples and not oranges.”

To set off elements that aren’t part of the main clause - “Of course, I agree.” If you can remove the extra element and still have a complete sentence with a complete meaning, then you should surround the extra words with commas. See Bracketing Commas for an overview. These extra elements include:

• Interrupting Words and Parenthetical Elements - “Commas are important, as you know.” Forms of direct address, weak interjections, disjuncts, conjuncts, and many common expressions are always set off with commas. See Commas and Introductory Words or Phrases.

• Nonrestrictive Elements - “My best friend, who I met at college, moved to England.” Nonrestrictive clauses and nonrestrictive appositives (Bob, my best friend, moved to England) are always set off with commas, just like all parenthetical elements.

• Introductory Phrases - “Somewhere in my messy closet, my old clothes are gathering dust.” With adverbial phrases (like prepositional and infinitive phrases), only use a comma if the phrase begins the sentence; if it follows the independent clause, don’t use a comma: “My old clothes are gathering dust somewhere in my messy closet.” See Commas After Introductory Phrases for more. Short prepositional phrases (and some other adverbial phrases) don’t require a comma — unless, of course, the meaning would be unclear (see Adding Commas for Clarity).

• Participial and Absolute Phrases - “Happily munching on popcorn, I watched my favorite movie.” When acting as adjectives, adverbs, or disjuncts, these types of phrases always require a comma, even if they follow the independent clause: “I watched my favorite movie, happily munching on popcorn.”

• Dependent Clauses - “If you help me, I’ll help you.” As with introductory phrases, if the dependent clause follows the independent clause, don’t use a comma: “I’ll help you if you help me.”

Other Conventional Uses - Used for clarity and convenience, commas also appear in names, addresses, numbers, and more:

• Places - “Austin, Texas.” See Commas with Geographical Names.

• Dates - “March 3rd, 2007.” See Commas with Dates. 

• Titles - “Martin Luther King, Jr.” See Commas with Titles that Follow Names.

• Addresses - “Post Office Box 555, Austin, Texas 55555.” See Commas in Addresses. 

• Greetings and Closings - “Dear President, [...]“ See Commas in Letter Writing.

• Numbers - “1,000,000 feet” See Commas in Numbers.

• Quotations - “She said, ‘Hello.’” See Commas with Quotations. For more on using punctuation marks with quotations, see Quotation Marks.

Semicolons

The semicolon is sort of like a cross between a comma and a period. Its main function is to connect two independent clauses that are closely related. It also prevents confusion in sentences (especially lists) with lots of commas. These websites explain everything you need to know:

• The Blue Book of Grammar and Punctuation - a short list of rules.

• UW-Madison Writing Center - another list of rules, along with some common errors.

• The Semicolon - an excellent overview, complete with lots of examples.

Colons

The colon introduces or restates something. Unlike the semicolon, the colon can connect an independent clause to a word or phrase.

• Armchair Punctuator - a complete overview with examples and usage notes.

• The Colon - another great overview.

• Sentence-Level Punctuation - a summary of the four ways a colon makes a restatement.

• The Colon - includes some usage guidelines, a discussion on when to capitalize the independent clause after a colon, and even a powerpoint presentation and a quiz at the bottom.

• Colons in Special Cases - a look at the colon’s minor uses.