3.2 Periods

Periods are used to indicate the end of declarative or imperative sentences:

The Newcomen engine utilizes the condensation of steam to create a partial vacuum beneath a piston.

Expand the system's bandwidth.

Failing to place a period after a complete sentence may produce a comma splice or fused sentence. Placing a period at the end of an incomplete sentence may produce a sentence fragment. In technical writing, exclamation points are not used except after warnings or caution statements.

In addition to ending sentences, periods have the following other functions:

A period is used after numbers or letters in an enumerated list.

Quantum theory has three main types of interaction:

1. strong

2. weak

3. electrical

End each item in an enumerated list with a period if one or more items in the list are complete sentences. For the sake of parallelism, normally all items or none should be complete sentences.

Sixty years of research have produced the following findings:

1. Low-calorie diets extend the life span of rats up to 30 percent.

2. The reduction of caloric intake is the only intervention that has demonstrably been shown to slow aging in rodents.

3. Caloric restriction initiated in mice at early middle age can extend the maximum life span by 10 to 20 percent and can oppose the development of cancer.

If a vertical list completes a sentence begun in the introduction to the list, the final period is omitted unless the items in the list are separated by commas or semicolons.

The status line contains the current

file name

window number

display modes

or

The mouse lets you

move around the spreadsheet;

position the cursor within a cell during Edit mode;

highlight menu items;

page through help screens.

Periods also stand for decimal points within numbers:

166.9

6.02

3.141516

Ellipsis points, three periods with spaces between and on both sides, are used to indicate omitted portions of a quotation:

Rolling inoculated cultures at 0.1 to 0.3 rpm may enhance viral isolations and can enhance CPE and viral yields for enteroviruses.

Rolling inoculated cultures at 0.1 to 0.3 rpm . . . can enhance CPE and viral yield for enteroviruses.

If the omitted portion follows a sentence period (or other end punctuation), the end punctuation before the ellipsis points is retained:

One can understand the spirit and characteristics of quantum electrodynamics without including this technical detail of polarization. But I’m sure you’ll all feel uncomfortable unless I say something about what I’ve been leaving out. Photons, it turns out, come in four different varieties, called polarizations, that are related geometrically to the directions of space and time.

One can understand the spirit and characteristics of quantum electrodynamics without including this technical detail of polarization. . . . Photons, it turns out, come in four different varieties, called polarizations, that are related geometrically to the directions of space and time.

Other punctuation marks that precede or follow the skipped portion are also retained for clarity and precision:

The earth's atmosphere is constantly shimmering because of moving pockets of air and changes in temperature, and such shimmering causes passing light rays to bend one way then another. In effect, the shifting atmosphere acts as a rapidly changing lens, smearing out and defocusing images.

The earth's atmosphere is constantly shimmering . . . . , smearing out and defocusing images.

A period is used after certain abbreviations, especially those that themselves spell words. If an abbreviation ending in a period comes at the end of a sentence, an additional period is not attached to the sentence:

The system shut down at 2:05 p.m.

Q.E.D.

Dr. Max Laue

Omission of a period in Mr, Ms, Mrs, or Dr is justified because these short forms are suspensions. Suspensions are shortened word forms that consist of the first and last letters of the full word, thus rendering the term “suspended” rather than simply abbreviated. In contrast, “Prof.” is not a suspension and should therefore be handled as a “true” abbreviation.

While the traditional Anglo-Saxon approach tends to adhere to the rule for suspensions (i.e., omitting the periods), international use of English has led to a more widespread acceptance of periods in social titles such as Mr. and Dr. In principle, you can decide on either style, as long as consistency is being maintained.

Similarly, you can use periods in academic titles or, alternatively, omit them. As scientific English evolves, periods tend to become less popular in shortened word forms (except in true abbreviations, e.g., temp.). There is no definitive rule, other than the need to be consistent.