The semicolon won't bite you
Today, National Punctuation Day, you can thank Aldus Manutius, the Italian printer who originated italic type and the octavo book, for the semicolon, which he is thought to have reintroduced in the late fifteenth century. He borrowed it from Greek, where it had been used as a question mark.*
Today it oddly stirs passions. Many writers disdain it, because they think it looks ugly or old-fashioned or pretentious. Hemingway seldom wrote a sentence long enough to require a semicolon, and the commas and dashes in Faulkner’s cascading sentences create hardly a ripple.
You want the full flavor of the semicolon, you have to go back to the nineteenth century. Here’s a random sentence plucked from Carlyle’s French Revolution: “Guillotining there was a Nantes, till the Headsman sank worn out: then fusillading ‘in the Plain of Saint-Mauve’; little children fusilladed, and women with children at the breast; children and women, by the hundred and twenty; and by the five hundred, so hot is La Vendée: till the very Jacobins grew sick, and all but the Company of Marat cried, Hold!”
Ah, they’re not writing semicolons like that anymore.
Today—and here you can proceed without fear—we mainly use the semicolon in two ways.
Indicating a complex series: If the elements of a series have items separated by commas, we use semicolons to separate the elements, including the final one: Some, a great many, eschew the semicolon out of principle; some, an insignificant minority, employ it out of principle; and some, by far the great majority, are ignorant of it and indifferent to it.
Suturing independent clauses: If we join two independent clauses without a conjunction, we link them with a semicolon: People who use the semicolon regularly are the type who like to parade their erudition; people who shun the semicolon are the type who like to be seen as just folks.
Two cautions here. First, remember that syntax implies relationship. If you join two clauses with a semicolon, they should be of more or less equal value in meaning, and they should have a connection in meaning.
Second, two independent clauses linked by a mere comma constitute the dread comma-splice run-on, the bugaboo with which generations of English teachers have frightened little children, to little visible effect. Linking independent clauses with commas is commoner than spam in written dialogue, and the practice is to be found among all sorts of British and American writers.
If you write a complex series, you are obligated to use semicolons. Apart from that, most people can shun them without ill effect. They will only come into play if you write with a particular kind of architecture, with a degree of complexity of elements and relationships that requires careful articulation of the parts. Should you embark on a sentence of that complexity, the semicolons will be the breadcrumbs that help readers find their way back.
*For more on the semicolon, including speculation on how the telegraph contributed to its drop in popularity, see Paul Collins’s excellent article in Slate from 2008.