This is from THE CRAFT OF PUNCTUATION (Based on J. Dawkins, “Punctuation as a Rhetorical Tool”):Punctuation marks are hierarchically ordered, based on the degree of separation they provide; for example, sentence-final marks (periods, exclamation marks and question marks) provide maximum degree of separation, whereas commas or no punctuation marks provide the minimum degree of separation. Colons, semicolons and dashes provide medium separation and have “extra” meaning--anticipatory, logical connection and emphatic, respectively.They left early. They were tired from the hike.They left early: they were tired from the hike.They left early; they were tired from the hike.They left early—they were tired from the hike.When writers use a period between two sentences, they are providing the highest degree of separation. The colon, semicolon and dash give a medium degree of separation and add meaning for the reader: the colon anticipates explanation—it indicates, in other words, that the clause following it explains the one preceding it; the semicolon lets readers know that there is a logical connection between the two clauses; and the dash makes the readers stop because the clause following it is emphasized. SourceThe following is from The Nuts and Bolts of College Writing (USA):
Dashes are a flexible and useful way of punctuating your writing—if not overused, they can give prose a punchy, vigorous feel. Dashes can act as semi-colons, connecting independent clauses. They can take the place of colons in introducing lists. They can serve to insert an example, and allow you gracefully to get back to the main sentence. You can also use them at any point in a grammatical structure when you feel a pause would be appropriate. Read more here
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