Despite what its name suggests, a semicolon isn’t exactly just half a colon—it has its own set of punctuational purposes. But since there is some overlap between the two marks, deciding which one to use can get tricky.
When to Use a Semicolon or Colon in a List
Both come in handy when you’re writing a list. A colon is used to introduce a list after an independent clause (i.e. a clause that can function as a full sentence by itself).
The band has three members: David Crosby, Stephen Stills, and Graham Nash.
As Grammarly explains, the colon in that sentence is working as a stand-in for the words who are. If the sentence started with “The band has three members, who are” or even just “The three members of the band are,” you wouldn’t need a colon (or any punctuation mark) before listing the musicians.
Semicolons, on the other hand, help split up items in a list that already includes commas or other punctuation (called a “complex series”).
The members of the band are David Crosby, a founding member of the Byrds; Stephen Stills, a founding member of Buffalo Springfield; and Graham Nash, a founding member of the Hollies.
If you replaced those semicolons with more commas, it might not be clear that the three phrases were describing the musicians—it might seem like each phrase was yet another member of the band.
When to Use a Semicolon or Colon to Join Two Sentences
Colons and semicolons can also both link two related sentences. If the second sentence is integral to the reader’s understanding of the first sentence—or you’re trying to highlight the second sentence—you probably want a colon.
Life is like a box of chocolates: You never know what you’re gonna get.
Without the second sentence, the meaning of the first sentence wouldn’t be obvious. So even though you can technically punctuate the first one with a period, it makes more sense to join them. (As for whether to capitalize the first word of the second sentence, it’s mostly up to you. Some sources recommend capitalization if it’s an independent clause, but it’s not necessarily wrong to leave it lowercase.)
If you just want to show that there’s a connection between two sentences, a semicolon is your best bet.
There’s too much uncertainty in a box of chocolates; I’d rather have one clearly labeled candy bar.
The second clause provides some nice additional information, but the first part doesn’t seem incomplete or confusing without it. That said, it’s ultimately up to you, the writer, to decide whether a colon or a semicolon fits your intention when joining two sentences.
In general, when you’re trying to decide whether to use a colon or semicolon, ask yourself this: Am I introducing information? If the answer is yes, choose a colon. If the answer is no, a semicolon might work (or a comma or an em dash, but we’ll save that can of worms for another lesson).