Semicolons and Colons

A semicolon is not, as it turns out, half a colon; but, as their names imply, semicolons and colons are in some ways closely related—and quite distinct in others.

Both can be used to join independent clauses, with somewhat subtle distinctions in effect. Both are also involved in the construction of lists, though they play different—and complementary—roles.

Semicolons

1. Semicolons can connect independent clauses; use of a semicolon indicates a close conceptual relation between discrete ideas:

Jean Cras orchestrated Polyphème while captaining a torpedo boat during the Adriatic Campaign of WWI; it was to be his only opera.

Semicolons therefore overlap in function with coordinating conjunctions; the effect, however, is different:

Semicolons therefore overlap in function with coordinating conjunctions, but the effect is different.

Note that a semicolon (or colon) is required when joining two independent clauses without using a conjunction, even when a conjunctive adverb is present; joining independent clauses with a comma alone is an error called a comma splice:

Yes: The international system is, in the end, anarchic: there is no overriding authority capable of arbitrating conflicts between states.

No: The international system is, in the end, anarchic, there is no overriding authority capable of arbitrating conflicts between states.

Yes: The international system is, in the end, anarchic; however, institutions can incentivize cooperation among states.

No: The international system is, in the end, anarchic, however, institutions can incentivize cooperation among states.

Just remember: a comma splice will not suffice.

(Opinions differ on whether it is acceptable to place a coordinating conjunction after a semicolon; so it’s up to you whether it’s worth risking stylistic censure in service of rhetorical effect.)
 

2. When a list is made up of items that themselves contain commas, semicolons clarify the list structure:

U.S. capitol cities include Sacramento, California; Lansing, Michigan; and Carson City, Nevada.

Without the semicolons, the boundaries between list items are muddier:

U.S. capitol cities include Sacramento, California, Lansing, Michigan, and Carson City, Nevada.

3. Semicolons separate multiple attributions in a single citation:

(Roussel 1912; Rimsky-Korsakov 1888)

Colons

The colon is a punctuation mark that says, in essence, “and here it is”: it announces that something just discussed is about to appear or be restated or further explained.

The relationship between the information on either side of the colon is therefore different from that created by a semicolon: while the semicolon indicates close relation, the colon generally signals identity.
 

1. Like the semicolon, the colon can join independent clauses, indicating reiteration or further development of a given idea:

Launch conditions were far from ideal: high wind speeds sent the balloon off course by 15 km, thereby compromising data collection.

A semicolon here would be technically acceptable but less conceptually precise: the statement that follows the colon defines the aforementioned adverse conditions rather than introducing a separate but related idea. Compare this sentence:

Launch conditions were far from ideal; further trials were therefore necessary to collect the required data.

2. Colons introduce lists, appositives (renaming / specification), and quotations:

• The remainder of this thesis is organized as follows: Chapter II provides . . . ; Chapter III describes . . . ; Chapter IV . . . .
• In this case, however, one instrument of national power overshadows the others: economics.
• Sun Tzu said it best: "Don't believe everything you read on the internet."

When using the colon in this way, be sure the introductory statement is a complete independent clause; that is, do not insert a colon where it will create an incomplete clause:

Yes: The DIME paradigm comprises diplomacy, information, military, and economics.

No: The DIME paradigm comprises: diplomacy, information, military, and economics.

Here, “comprises” needs an object(s), so cutting off the syntax with a colon separates sentence components that need to be grouped together in the clause.

3. Finally, colons perform assorted functions in constructions related to time, mathematics, and citation styles.

More Information on Semicolons and Colons

• GWC writing mechanics module: Colons & Semicolons
• Webpage: "How to Use a Semicolon," The Oatmeal
• Article / video (0:29): "How to Use Semicolons," Grammar Girl
• Video (4:38): "Run-ons and comma splices," Khan Academy
• Website: The Punctuation Guide