Periods with Other Punctuation Marks

A period is placed inside quotation marks:

In Physics and Reality, Einstein wrote, "The whole of science is nothing more than a refinement of everyday thinking."

A period is placed after the parenthesis or bracket at the end of a sentence (unless the parentheses or brackets enclose a compete new sentence).

One such by-product is the superoxide radical (O₂^·-).

[The dot in the formula represents the unpaired electron.]

3.3 Commas

Commas help to keep the writing clear. Omitted or incorrectly used, commas can cause confusion or even change meaning. Commas should be used with the following elements:

- introductory elements

- coordinating conjunctions joining independent clauses

- elements in a series

- coordinate modifiers

- non-defining modifiers

- parenthetic elements

- elliptical constructions

- commas may also have other specialized uses.

1. Introductory Elements

Commas are used to set off transitional words and phrases, introductory clauses, or introductory phrases, to signal where the introductory element finishes and the main part starts.

Transitional Words and Phrases

Moreover, the opening of an export market would help expand the market for key escrow encryption.

In addition, several companies and individuals have proposed commercial key escrow approaches.

Introductory Clauses

A comma is placed after an introductory dependent clause. However, if the subordinate clause comes after the main clause, you need no comma:

Although key escrow is voluntary, critics say that the introduction of Clipper points national policy in a disturbing direction.

When the liquid boils, a residue is formed.

A residue is formed when the liquid boils.

Introductory Prepositional or Verbal Phrases

Normally, a comma is used after an introductory prepositional or verbal phrase. However, you may omit the comma after a short introductory phrase if no ambiguity is possible:

For the first time, researchers have used DNA analysis to identify the animal tissue in 4,000-year-old rock paintings.

Despite the error the experiment was successful.

Combining surface area with depth, we calculated the volume of the pond.

NB! A comma is not placed after an introductory participial or gerund phrase if the phrase forms part of the subject or verb of the sentence:

Combining surface area with depth was our principal method for calculating the volume of the pond.

2. Coordinating Conjunctions Joining Independent Clauses

A comma is placed before a coordinating conjunction (and, but, so, or) that joins two independent clauses. However, if the clauses are very short and closely related, you may omit the comma:

Incorrect

In almost all illicit markets, only the tip of the iceberg is visible and there is no reason why the nuclear-materials black market should be an exception.

Correct

In almost all illicit markets, only the tip of the iceberg is visible, and there is no reason why the nuclear-materials black market should be an exception.

3. Elements in a Series

A comma is used to separate items in a series. Although placing a final comma before the coordinating conjunction is often considered optional, omitting it can sometimes cause confusion. Consequently, most scientific and technical writing routinely uses a final comma in a series to prevent possible ambiguities:

His intellectual acuity, diverse interests, frail physique, and ethereal personality made Oppenheimer a man of legendary proportions. [In this sentence, omitting the final comma would not cause confusion. Still, it is good practice in scientific and technical writing to always include the final comma in a series.]

Weak

Contemporary physics is still exploring neutron stars, black holes and the penetration of electrons through potential barriers. [The omission of the final comma before and causes ambiguity: are black holes and the penetration of electrons through potential barriers specific topics connected to the general subject of neutron stars, or are they separate items?]