Semicolons, colons, and commas

When to Use a Comma

To list items or short phrases:

I bought bread, cheese, and pickles at the grocery store.

To separate long independent clauses when conjunctions connect them:

Astronomers have known about the positions of stars for centuries, but they didn’t understand that the earth revolves around the sun.

After an introductory phrase:

In preparation for the next convention, the representatives studied up on the most important issues.

To separate a parenthetical phrase or interrupter:

All doctors, if they care about their patients, are concerned with good office hygiene.

Common Comma Mistakes

Comma splice. Do NOT use a comma to divide two independent clauses without a conjunction.

Incorrect: Thousands of protesters showed up on the streets, they were shouting and carrying large posters.

Correct: Thousands of protesters showed up on the streets; they were shouting and carrying large posters.

Combining lengthy phrases. Using commas with compound sentences or to separate clauses with other commas can cause confusion. Use a semicolon.

Incorrect: Some useful subjects are English, which is an international language, math, which is used in all domains of sciences and social sciences, and philosophy, which underpins many other areas of study.

Correct: Some useful subjects are English, which is an international language; math, which is used in all sciences and social sciences; and philosophy, which underpins many other areas of study.

When to Use a Colon

A colon is used to make lists and tell the reader, “This is what I mean.”

To introduce an item or series of items:

Humans use five major senses: sight, hearing, smell, taste, and touch.

To separate independent clauses when the second clause/sentence illustrates, explains, paraphrases, or expands on the first:

Martha realized that her worst fear was coming true: her son was being sent to war.

*The information after the colon explains Martha’s fear in the form of an independent clause.

To follow the salutation in a business letter or more formal letter:

To the Central Valley Committee Chairman:

More Tips For Using Colons

Do not capitalize the first word after the colon unless it is a proper noun, is part of a quote, or is the first in a series of sentences:

Incorrect: I have three desires: To eat, sleep, and work.

Correct: I have three desires: to eat, sleep, and work.

When a quotation contains multiple sentences, many writers prefer to introduce it with a colon rather than a comma:

In Chapter 3, the author explains his theory: “Dogs have dreams, but they don’t dream as humans do. Their dreams reflect a primal desire for pleasure, whereas humans are preoccupied with the ego and self-image. This is equally true in wakefulness and sleep.”

A complete sentence after the colon is not necessary—a word or phrase is fine.

There is one mantra that can sum up our position towards climate change: urgent action.

Common Colon Mistakes

Separating two clauses that have equal rank or unrelated information:

Incorrect: Sarah and her friends loved spending time on the mountain: nature always held a special meaning for them.

Correct: Sarah and her friends loved spending time on the mountain; nature always held a special meaning for them.

*Use a period if the clauses are not at all related. Use a semi-colon if the information in the second clause is somewhat related but does not illustrate, explain, or paraphrase the first clause.

Overusing the colon:

The colon is a powerful punctuation mark and should be used sparingly. Think of it as a stop sign that calls readers’ attention and says, “Hey! Pay attention to this. This is important.” If there are too many stop signs on a street, you won’t be able to drive very smoothly or quickly. This is the same effect colons can have upon readers.

When to Use a Semicolon

A semicolon is used to separate two ideas (two independent clauses) that are closely related. They can also be used when listing complex ideas or phrases that use commas within them.

To join two or more ideas (parts) in a sentence when their ideas are given equal position or rank:

The universe has always called to human beings; there could be no more final frontier than space.

To join two independent clauses connected by conjunctive adverbs or transitional phrases:

Sam thought David was inviting him to the picnic to enjoy a nice day out; as it turned out, David was planning a surprise birthday party.

To present items in a list or series if there are commas within the items, or if the items are relatively long and complex:

Our family members came all the way from Denver, Colorado; Rochester, Minnesota; and even Paris, France.

To connect independent clauses joined by a coordinating conjunction (if the clauses are already punctuated with commas) or if they are lengthy.

My main research objective is to isolate the cause of the disease, as well as to contribute to the existing literature; for this will bring an end to starvation across the continent, create new study designs related to epidemiology, and change the very paradigm of my research field.

Using a Semicolon Instead of a Comma

Incorrect: The specimens were treated properly, however, they were not stored properly.

Correct: The specimens were treated properly; however, they were not stored properly.

*The conjunctive adverb “however” shows a connection between the two independent clauses; do not use a comma to connect two independent clauses without a coordinating conjunction (and, but, or, nor, etc.).

Incorrect: The sun is wonderful: it produces light, which plants need to survive, it gives us warmth, which is useful for most life, and it makes a sad day happier, which is obviously a positive trait!

Correct: The sun is wonderful: it produces light, which plants need to survive; it gives us warmth, which is useful for most life; and it makes a sad day happier, which is obviously a positive trait!

*Using only commas, it is unclear what items are being listed exactly. Semicolons help divide listed items that each contain commas.