Here are a few notes about punctuation. This article can end many many long pointless conversations, so keep it at hand. I have not written a single line in this article. It is all copy-pasted from other sources I cannot quite recall right now.

1. Comma splices (commas used incorrectly between two complete sentences or main clauses, as in I just love those commas, they’re so much fun; instead use periods, semicolons, dashes, or conjunctions between complete sentences); especially with:

• however (as in I know what I know, however, I don’t know what you know)
• therefore (as in She knows the rules, therefore, she makes few errors)

“Then, however, thus, hence, indeed, and therefore are adverbs rather than conjunctions and should be preceded by a semicolon [or a period] when used as a transition between the clauses of a compound sentence” (U of M Rule 2, p. 33).

1. Should I use a comma before and in a series?

It’s your choice. Most business writers prefer to use the “serial comma” before and in a series of three or more items. For example, “I like carrots, broccoli, and succotash.” Most newspapers, however, omit the comma before and. Whatever your preference, be consistent. Also, be sure to use the style of the publication or organization for which you are writing. (Rule 162, p. 35; also, see The Associated Press Stylebook and Libel Manual, p. 269.)

2. Should I use a comma when the conjunction and joins two clauses in a sentence?

Use a comma when and joins two clauses consisting of two subjects and two verbs; omit the comma when and joins two verbs that have the same subject. For example, “She wrote the memo, and she sent it,” but “She wrote the memo and sent it.” (Rule123, p. 14). Note: The comma in the first example may be omitted when the clauses are short.

3. When should I use commas with that and which?

Introduce a restrictive or essential clause with that and do not mark it with a comma or commas. For example, “Sally wrote the report that was approved. Susan wrote the report that was rejected.” Introduce a nonrestrictive or nonessential clause with which and mark it with commas. For example, “Sally’s report, which was approved, was the only one considered.” (Rule131, p. 19, and Rule1062, p. 237)

4. Should I use commas before and after the year in a date?

Yes. Use two commas to set off the year when it follows the month and day and the sentence continues, as in “August 15, 1997, was a scorcher.” But note: “August 1997 was an unusually warm month.” (Rule154, p. 33)

5. What is a “comma splice,” why is it a problem, and how can I avoid it?

A “comma splice” occurs when two complete sentences are joined (or spliced together) with a comma rather than with a concluding punctuation mark such as a period or a semicolon. In informal writing such as popular fiction, comma splices are common (especially in dialogue), but in on-the-job writing (business writing, technical writing, legal writing, academic writing, journalism, etc.) they are considered a distracting error. Readers generally want to know where one sentence ends and the next begins. The most common comma splice occurs when the adverb however is mistaken for a conjunction. For example, change “I only wanted to help, however, I hurt his feelings” to “I only wanted to help; however, I hurt his feelings” or to “I only wanted to help, but I hurt his feelings.” When two complete sentences are linked by a transitional expression such as however and therefore, use a semicolon – not a comma – between the sentences; use a comma after the transitional expression. (Rule178, p. 40)

6.  Should I use one space or two after periods and colons?

The trend is to use only one space after these marks, although some style manuals still call for two.  The main thing is to space consistently throughout a document.

In his widely used reference guide, The Gregg Reference Manual, William Sabin calls for two spaces. But in The Mac Is Not a Typewriter, Robin Williams (not the actor) calls for one. The argument for one space is that kerning, or spacing between words, automatically allows for a little extra space after periods and colons.  Note that most e-mail software will transmit only one space anyway (perhaps to minimize transmission time).
Nevertheless, because I think the extra white space opens the text and makes it look more inviting to the reader, my personal preference is for two spaces. I suppose I’ll eventually give in and use just one space.

7. Do commas and periods go inside or outside the closing quotation mark?

According to American usage (which differs from the “British style”), commas and periods go inside the closing quotation mark. Other punctuation marks (such as question marks, colons, and semicolons) go inside or outside, depending on whether they are part of the quoted material.
Some writers and language experts argue that commas and periods, when not part of the quoted material, are more logically placed outside the closing quotation mark. Standard American usage, however, is based on the notion that placing commas and periods within the closing quotation mark causes no significant confusion or ambiguity.

Here is how The Chicago Manual of Style (14th ed., pp. 160-61) describes the rationale behind its position:

Periods with Quotation Marks

American Style

When a declarative or an imperative sentence is enclosed in quotation marks, the period ending the sentence is, in what may be called the American style, placed inside the closing quotation mark. If the quoted sentence is included within another sentence, its terminal period is omitted or replaced by a comma, as required, unless it comes at the end of the including sentence. In the latter case, a single period serves both sentences and is placed inside the closing quotation mark.

“There is no reason to inform the president.”

“It won’t be necessary to inform the president,” said Emerson.

Emerson replied nervously, “The president doesn’t wish to be informed about such things.”

British Style

The British style of positioning periods and commas in relation to the closing quotation mark is based on the same logic that in the American system governs the placement of question marks and exclamation points; if they belong to the quoted material, they are placed within the closing quotation mark; if they belong to the including sentence as a whole, they are placed after the quotation mark. The British style is strongly advocated by some American language experts. In defense of nearly a century and a half of the American style, however, it may be said that it seems to have been working fairly well and has not resulted in serious miscommunication. Whereas there clearly is some risk with question marks and exclamation points, there seems little likelihood that readers will be misled concerning the period or comma. There may be some risk in such specialized material as textual criticism, but in that case author and editors may take care to avoid the danger by alternative phrasing or by employing, in this exacting field, the exacting British system. In linguistic and philosophical works, specialized terms are regularly punctuated the British way, along with the use of single quotation marks. With these qualifications, the University of Chicago Press continues to recommend the American style for periods and commas.

Use quotation marks to:

• Mark direct quotations, as in He said, “Try your best.” (Note that the first word of the quote begins with a capital letter.)
• Mark titles of shorter works, such as articles, poems, and chapters. (Use italics or underlining for titles of longer works, such as books, plays, and films.)
• Call attention to a word, phrase, or concept that is unfamiliar to the reader or that is used in a nonstandard way, as in Based on empathy rather than confrontation, “Rogerian persuasion” offers an alternative to classical argumentation.
• Call attention to a nontechnical term used in a technical sense, as in Deconstructionism explores the meaning of the “signs” of language.

Do NOT use quotation marks to:

• Mark indirect quotations or paraphrases, as in Our boss said that we should persevere. (But Our boss said, “Never say die!”). Note that no comma is used to mark a paraphrase after the word that.
• Mark a cliché, proverbial saying, or other overused expression, as in “Quality control” is our strength, or We need to do “our very best.”
  Sometimes called “winking,” this last example reflects a tendency for writers to disown or apologize for worn-out language. Although the quotation marks are intended to convey “I know this is lazy wording I could have done better, but I didn’t have time,” in reality they tell the reader “If you were more important, I would have taken time to find more appropriate wording but you aren’t, and I didn’t.”
  If you are going to use a familiar word or expression, do so without apology.
• Emphasize a particular word or phrase. Instead, use italics, as in “I am absolutely certain.”
• Mark yes and no when used alone, as in She said yes.
• Mark a word or phrase after the expression so-called, as in She is the so-called leader of the group (not She is the so-called “leader” of the group).

Miscellaneous rules and conventions:

• Use single quotation marks only for quotes within quotes and for headlines in newspapers and publications.
  Avoid the common error of placing single quotation marks around words or phrases and reserving double quotation marks for use around complete sentences.
• Place commas and periods inside quotation marks.
• Place semicolons, colons, and question marks outside quotation marks, unless they are part of the quotation. Here, American usage follows the logic of the British system, perhaps because these punctuation marks are considered more obtrusive than commas and periods.
• If the quotation ends with a question mark or exclamation point, omit the first comma, as in “Do you want me to do this first?” Sally asked; not “Do you want me to do this first?,” Sally asked. And not “Do you want me to do this first?”, Sally asked.
• For a quotation longer than one paragraph that is not set off from the text in a block, use quotation marks at the beginning of each paragraph and at the end of the final paragraph.
  For example, in a three-paragraph quotation do not use quotation marks at the end of the first and second paragraphs. Leave those paragraphs “open” to indicate that the quotation continues.
• If a one-sentence quotation is interrupted by a phrase of attribution or signal phrase, use quotation marks around both parts of the quotation, a comma after the first part of the quotation, and a comma after the phrase of attribution, as in “I’ll finish this project,” Sally said to her boss, “as soon as I get the figures from accounting.”
• If a two-sentence quotation is interrupted between the sentences by a phrase of attribution, use quotation marks around both sentences, a comma after the first sentence of the quotation, and a period after the phrase of attribution, as in “I will finish this report on time,” Sally assured her boss. “It will be on your desk by 2 p.m.” (Note that the first word of the second quoted sentence begins with a capital letter.)
• Set off long quotations as blocked or indented text. Long quotations generally are defined as having more than 4 lines. (Some style manuals define long quotations as having more than 40 words; The Chicago Manual of Style defines long quotations as having more than 10 lines.) The sentence introducing a long quotation ordinarily ends with a colon (rather than a comma).
• Present block quotations without quotation marks. (Use quotation marks as you normally would for quotations within the block quotation.)

Style manuals differ on whether block quotations within a double-spaced document should be single-spaced or double-spaced, and whether they should be indented from the left side margin only or from both side margins. The Chicago Manual of Style and other academic style manuals call for double-spacing and indenting from the left side margin only; The Gregg Reference Manual, a standard guide for business writing, calls for single-spacing and indenting from both side margins.  To on-the-job writers, I recommend single-spacing and indenting from the left side only.

Quotation marks were invented to make life easier. Those neat little squiggly marks provide an unobtrusive way to indicate a change in voice. They help readers keep track of who’s doing the talking.

Four guidelines to help keep you out of trouble

• Use single quotation marks only for quotes within quotes and for headlines in newspapers and publications. Avoid the common error of placing single quotation marks around words or phrases and reserving double quotation marks for use around complete sentences.
• Place commas and periods inside quotation marks. For a discussion of the “American style” (in which commas and periods are placed within quotation marks) versus “the exacting British system” (in which commas and periods are placed either inside or outside quotation marks depending on whether they are part of the quotation), see The Chicago Manual of Style (14th ed.), pages 160-61, or see Punctuation: FAQ.
• Place semicolons, colons, and question marks outside quotation marks, unless they are part of the quotation. Here, American usage follows the logic of the British system, perhaps because these punctuation marks are considered more obtrusive than commas and periods.
• For a quotation longer than one paragraph, use quotation marks at the beginning of each paragraph and at the end of the final paragraph. In a three-paragraph quotation, for example, do not use quotation marks at the end of the first and second paragraphs. Leave those paragraphs “open” to indicate the quotation continues.

by Xananax