Interruptive Punctuation
Have you ever wondered when to use a semi-colon, or what the difference is between comma-delineated text and parenthesis-delineated text? Here's a short list.
Period: This is a full stop. A period is used only after complete sentences with one exception: it can be used in lists to separate fragments where commas or semi-colons do not make sense. Do not use a period after list elements that stand alone and are not complete sentences.
Semi-colon: A semi-colon provides a partial stop; a semi-colon is used to separate equally grammatically complete (or, rarely, equally incomplete) parts of a sentence that are related in topic. Use semi-colons to "soften" the stop compared to a period. Practically speaking, you don't need these very often, as a period typically suffices. Semi-colons can also be used to separate elements in a list if you need interior punctuation within the elements of the list.
Colon: A colon provides a partial stop but does not imply a complete sentence cumulatively or on either side. Use colons to offset phrases, like in a title, prefacing bulleted or tabular lists, or to introduce a long quotation. Typically, the part before the colon introduces a topic and the part that follows clarifies or defines in some way. Colons are seldom part of complete sentences.
Comma: A comma is required after prepositional phrases in American English (not British), is used when making lists or deliberately imposing a slight pause, and separates out optional bits of information, such as the dreaded caveat words. Use commas when the comma-delineated section is something that the reader should read, but that could be removed without tragedy to the understanding borne by the sentence, and leaving the sentence grammatically intact.
Parenthesis: Parentheses present entirely optional material. You (can't resist, but you) include the material for the edification or pleasure of the reader. The structure of the sentence should flow as if the parenthetical bit were removed. If the contents of your parenthesis can't be removed and still do right by your reader, it doesn't belong in parenthesis; either bump it up to comma delineation or make it into its own sentence.
Em-dash (en-dash, hyphen): Em-dashes are reserved for breathless interruption. The em-dash-delineated material really has no impact on content--you just have to say it or you'll burst. The structure of the surrounding sentence, like commas and parenthesis, should be as though the em-dash-delineated section did not exist. If your content should not be removed under any circumstances (by over-eager editors, perhaps), it does not belong between em-dashes. You may use one em-dash, also for breathless interruptions. Em-dashes are not used to separate elements in tabular lists from their definitions. Sadly, you see this on the Internet, but it is wrong. (Period. Full stop. It is wrong.) Em-dashes are so named because they are as wide as the old typesetter's "M." Their relatives, the en-dash (as wide as an "N" and used to show a spread of numbers, like 9-12) and the hyphen (used erroneously as a short em-dash in documents for whom the lazy AP Press manual of style is acceptable, and correctly used to link related words and parts of compound words), are not substitutable for the em-dash. Em-dashes have no spaces surrounding them, and neither do en-dashes or hyphens. The evil and bad hyphen-as-em-dash does have spaces surrounding it, compounding its wrongness three-fold.
Ellipsis: This series of three periods (or four, if it ends a sentence) illustrates either an omission from quoted material or a trailing off of thought or speech in poetry or fiction. Like exclamation points, there is little demand for such artistry (or exuberance) in technical text.
I'll do another blog on question marks, exclamation points, brackets, and slashes. These points of punctuation are more controversial, or at least I have opinions about them that might be controversial.
(For those of you wondering what organization principle I used here, it was “degrees of stoppage.” It just made more sense than alphabetical.)