Punctuation Basics — Commas and Semicolons
by Marci Bethel

When to Use Commas

This list covers some common reasons to use commas. There are others, but these are the ones that writers typically have trouble with.

To enclose…

• nonrestrictive clauses (not essential to the meaning of the sentence; which clauses)

The register editor, which is a fine piece of software, configures registers.

• parenthetical elements

The ApBUILDER, of course, is a fine piece of software.

• appositives (phrases that identify another expression)

Our company, Intel Corporation, manufactures semiconductors.

• nonrestrictive participial phrases (a participle is a verb form that acts as an adjective)

Robin, working efficiently, finished the project early.

To separate…

• independent clauses (those that could stand alone as complete sentences) linked by a coordinating conjunction (and, or, nor, but, for, so, yet)

The register editor configures registers, and the instruction editor composes code.

• contrasting ideas

We finished the project on time, but over budget.

• introductory elements

Since typos seem to creep into all of our work, it is wise to proofread everything twice.

During the development cycle, we read everything a million times and still miss typos.

• items in a series

Set the YES bit, the MAYBE bit, or the NO bit.

• adjectives describing the same noun or phrase

The article recommends using this new, state-of-the-art, useful software product.

• conjunctive adverbs (however, nevertheless, therefore, consequently, thus, hence, besides, for example, on the other hand) that join two independent clauses. A semicolon precedes a conjunctive adverb and a comma follows it.

Your idea is good; however, we can't afford to do it now.

Your idea is costly to implement; consequently, we can't do it.

When Not to Use Commas

This list describes some common pitfalls. (* represents a place where commas are sometimes used in error.)

Don't enclose…

• restrictive clauses (essential to the sentence; that clauses)

The software* that configures registers* is the register editor.

Don't separate…

• the two halves of a correlative conjunction (not only … but also, either … or, neither … nor, both … and, whether … or)

The register editor not only configures registers* but also configures peripherals.

• two-element subjects

ApBUILDER* and DemoBUILDER boosted sales.

• two-element predicates (a predicate is a verb plus its object)

ApBUILDER boosted sales* and increased profits.

• two-element objects

ApBUILDER configures the device registers* and the related peripherals.

• a subject and its verb

The people who develop ApBUILDER* work long hours.

• a verb and its object

The manager reported* that productivity had increased.

• the subject from the rest of the sentence

To be able to finish ApBUILDER* is the goal of the team.

Working on the weekends* will help us do that.

When to Use a Semicolon

• To join independent clauses without a conjunction.

The register editor configures registers; the instruction editor writes code.

• To separate a transitional element (typically a conjunctive adverb — however, nevertheless, therefore, consequently, thus, hence, besides, for example, on the other hand) that joins two independent clauses. Use a semicolon before and a comma after the transitional element.

We proofread everything; however, we still miss some errors.

• To separate phrases or clauses in a series when one or more of the phrases or clauses contain commas.

ApBUILDER has online, hypertexted manuals; whizzy, useful design editors; and lots of other terrific features.

References:
Brusaw, Alred, and Oliu, Handbook of Technical Writing, 2nd ed. New York: St. Martin's, 1982
A Manual of Style. 13th ed., rev. Chicago: The University of Chicago Press, 1982.
Alley, Michael, The Craft of Scientific Writing, Englewood Cliffs, New Jersey: Prentice-Hall, Inc.

Printer-friendly (PDF) version of this page.