Connecting Two Independent Clauses
An independent clause is a clause with at least one subject and one verb that can stand on its own as a sentence. That means independent clauses can end with a period and still be grammatically correct. The independent clause can also be connected to dependent clauses or other independent clauses to form longer sentences. When connecting two independent clauses you can use a comma with a conjunction or a semicolon. When you choose to use them is a decision of style, so let's look at a few examples.
'Cecilia's favorite animal was the narwhal. She loved it mostly for its unicorn horn.'
'Cecilia's favorite animal was the narwhal, and she loved it mostly for its unicorn horn.'
'Cecilia's favorite animal was the narwhal; she loved it mostly for its unicorn horn.'
Each of these sentences is perfectly correct grammatically, but the way they appear on the page - more than the way they sound when I read them - makes a different impact on the reader. In the first example, there's a definitive break making the two ideas separate, even if they're thematically connected. In the second example, the conjunction actually makes the sentence feel a little stiffer, as if the writer is trying to jam the two ideas together. In the third, where the semicolon comes into play, it reads aloud basically the same as the first sentence but without the full stop of the period and subsequent capitalization of the word 'she.' So, the relationship between the two clauses is much chummier. That is, they seem like they belong together. Technically, you can always use a semicolon to unite two independent clauses, but you should really only use it when you want to suggest this kind of specific intimate relationship.
Connecting Transitional Words and Phrases Within a Sentence
Other uses of the semicolon are more straightforward, but the same guideline applies. For instance, you have the option of using a semicolon when connecting two independent clauses with a conjunctive adverb- that is, words, like 'however,' moreover,' 'therefore,' or phrases like 'for example' - that serve to transition and further explain something about the main clause.
Here's the version that you usually see, with the conjunctive adverb starting the next sentence:
'Grover loved to cook. However, he was terrible at it.'
But if you want to strengthen the relationship between two clauses, a semicolon can stand in for the period.
'Grover loved to cook; however, he was terrible at it.'
A semicolon is better suited here than a period because Grover's cooking ability is intimately related to his love of cooking.
Keeping Items in a List That Have Internal Punctuation Separate
This final use of the semicolon is the easiest, but also has a tendency to confuse people when they see it in text. The simple guideline is this: you know how you use commas to separate items in a list? As in, 'For breakfast, the family had waffles, eggs, orange juice, toast, and hash browns.' That works perfectly well as long as none of the items in the list have their own internal punctuation. If that's the case, it can get confusing, which you'll see if we change this sentence just a little.
'For breakfast, the family had waffles, sausage, egg, and cheese biscuits, orange juice, toast, and hash browns.'
Now, it's unclear whether sausage, egg, and cheese biscuits are three separate items, or if egg and cheese biscuits are one item, or if sausage, egg, and cheese biscuits are one entire item. To clarify, you use a semicolon.
'For breakfast, the family had waffles; sausage, egg, and cheese biscuits; orange juice, toast, and hash browns.' It's now clear to the reader that sausage, egg, and cheese biscuits are one breakfast item. Always use a semicolon to clarify items in a list whenever you think there could be this kind of confusion.