There are three forms of punctuation that offset one portion of a sentence from the rest, colons, semicolons, and em- dashes. Each has a specific function and knowing the rules can help you write clearer interruptions.

Colons

Colons are most often used for definitions. One side of the colon has the word or phrase to be defined, and the other side contains the definition or clarification. Use a colon in place of a verb or the words “for example,” and when both sides of the colon contain fragments (incomplete sentences). The keys to good cooking: fresh ingredients and simplicity. Three of my favorite early composers: Josquin, Perotin, and Palestrina. Getting Smart: Using Purple People Plotters. Don’t use a colon between a verb and its object. The keys to good cooking are fresh ingredients and simplicity. Three of my favorite early composers are Josquin, Perotin, and Palestrina. Use a colon when you provide a list of definitions (but not in a table). Lots of folks use em-dashes in lists, but as you’ll read in a minute, em-dashes aren’t appropriate in such a use. Early Music: a classification of western, or European, music from before 1750 A.D. Contemporary Music: a classification of western, or European, music within 50 years of the present period. A capital letter follows the colon only when the word is a proper noun, when the phrase is part of a title, or when what follows is a complete sentence. It’s optional to capitalize the complete sentence (the third instance), considered a fairly “modern” usage. Early composers: Josquin, Perotin, and Palestrina. Getting Smart: Using Purple People Plotters for Productivity Plusses We are at an impasse: We must fight or flee. Use a colon to introduce lengthy quoted material. The quoted material can be part of the same sentence and paragraph as the introduction, or it can precede a paragraph (or several) of quoted material. You don’t need quotation marks before a long quotation. (Short quotations need a comma and quotation marks, no colon.) The clever editor said: It is important not to distract from what you’ve written by how you’ve written it. (This sentence could comfortably also go the way of a comma and quotation marks.) The clever editor said: It is important not to distract from what you’ve written by how you’ve written it. Whatever follows a colon refers back to what immediately preceded it. Be sure that in a list, each element is grammatically consistent with the lead-in and colon. There are three main points: The top of the Purple People Plotter must be bare. The plug must be firmly connected to an electrical source. The object to be plotted must be renderable in purple. Notice that there are periods after those list elements: Periods belong only after complete sentences. If your list elements are not complete sentences (not including the lead-in), don’t use a period. Each of the elements should make a complete sentence in context of the lead-in. Be sure that each of your elements matches: they should either all be complete sentences or none of them should be. If you’re restating or clarifying a point, use a colon. Use a semicolon to make a new point.

Semicolons

Use a semi-colon to separate two grammatically equal sides of an expression. You could almost think of a semi-colon as replacing a conjunction. You use a semicolon when you want a full stop and want the second sentence to be visibly linked to the first, much like the effect of a conjunction. Three plots made by the Purple People Plotter were lavender; the staff liked the color and were loathe to report the incident as an error. Three plots made by the Purple People Plotter were lavender but the staff liked the color and were loathe to report the incident as an error. You also use a semicolon to separate elements in a list that have other forms of punctuation. The semicolon helps keep the specific elements separated and allows the interior sections to be comfortably punctuated. The concert contained music by Palestrina, a 16 th century Italian; Josquin, a 15 th century Belgian; and Brahms, a 19 th century German. Do not use a semicolon to link unrelated ideas. This, for instance, is wrong: The man was a sports nut; there were four flowers in the vase. If the sentences in your piece are all uniformly short, you can link some of them with semicolons to keep things interesting. Be careful not to over-use them. You’ll look like you never really finish anything if you do.

Em-dashes

Em-dashes (a double hyphen) are used to interrupt and are not used to offset phrases or definitions. Use an em-dash when you want to include a certain breathlessness into your sentence. Three plots made by the Purple People Plotter were the wrong color—the staff loved it. Use two em-dashes, one on each side of an expression, to set aside completely unnecessary parenthetical information that you are too excited to suppress. Music will be performed in several languages—French, Italian, Latin, and German—at Saturday’s concert. Interruptions are a necessary part of keeping the text interesting. Do keep them to a minimum, unless you want to sound like you can’t ever finish a thought.