Use periods 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, do not use exclamation points except after warnings or caution statements. (See Exclamation Points.)
In addition to ending sentences, periods have the following other functions.
Quantum theory has three main types of interaction:
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
The mouse lets you
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.
--Taniya Sarkar, Wei Zhao, and Nurul H. Sarkar, "Expressions of jun Oncogene in Rodent and Human Breast Tumors," World Wide Web Journal of Biology
If the omitted portion follows a sentence period (or other end punctuation), retain the end punctuation before the ellipsis points.
One can understand the spirit and characteristics of quantum electrodynamics without including this technical detail of polarization. But Iím sure youíll all feel uncomfortable unless I say something about what Iíve 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.
----Richard Feynman, QED
Retain other punctuation marks that precede or follow the skipped portion 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
The earth's atmosphere is constantly
shimmering . . . . , smearing out and defocusing
--Alan Lightman, Ancient Light
Dr. Max Laue
If an abbreviation ending in a period comes at the end of a sentence, do not attach an additional period to the sentence.
The system shut down at 2:05 p.m.
In Physics and Reality, Einstein wrote, "The whole of science is nothing more than a refinement of everyday thinking."
Place a period 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
--Richard Weindruch, "Caloric Restriction and Aging," Scientific American