Before I discuss comma splices I will briefly explain run-on sentences, since there is some overlap in definition. A run-on sentence – also known as a fused sentence – is a sentence in which two or more independent clauses run together, i.e. the clauses are not joined by a conjunction or by punctuation:
The tests were inconclusive I didn’t know what to do next.
Everyone was ready however there were unforeseen delays.
Such sentences don’t stop, or pause properly, when they should. Neither do sentences with a comma splice, which occurs when two or more independent clauses are joined only by a comma:
The tests were inconclusive, I didn’t know what to do next.
Everyone was ready, however there were unforeseen delays.
Some grammarians consider comma splices as a type of run-on sentence; others distinguish them but discuss them together.
The comma splice is also called a comma error, comma blunder, and comma fault, but I find these terms too judgemental. Comma splices can be fine in fiction, poetry, letters and informal writing in general, where they often reflect spoken English and join clauses that are short, connected by subject or content, and unlikely to be misconstrued:
It wasn’t broken, it needed new batteries.
The shops were all closed, I couldn’t buy milk.
Commas are weak marks: they can separate elements within a clause, but they are not always considered strong enough to separate independent clauses. Comma splices (and run-on sentences) can draw readers into a second independent clause before they know that the first one is finished.
It is therefore preferable in many kinds of formal writing to separate such clauses with a conjunction or a stronger punctuation mark: a colon, semicolon, full stop or dash will supply the necessary pause. Simple rearrangements are another option.
It wasn’t broken, but it needed new batteries.
It wasn’t broken – it needed new batteries.
The shops were all closed, so I couldn’t buy milk.
As the shops were all closed, I couldn’t buy milk.
The shops were all closed; I couldn’t buy milk.
I couldn’t buy milk because the shops were all closed.
The tests were inconclusive, and I didn’t know what to do next.
Since the tests were inconclusive, I didn’t know what to do next.
The tests were inconclusive; I didn’t know what to do next.
The tests were inconclusive. I didn’t know what to do next.
All of these revisions are fine, and you can probably imagine many others. Which approach you choose depends on what suits the context, what tone and rhythm you want to convey, and so on.
Comma splices were more common in 18th and 19th century English, when they were not considered ungrammatical. Although modern English is more rigorous, comma splices have been used by authors like William Faulkner, Samuel Beckett, E. M. Forster, John Banville, Iris Murdoch, E. L. Doctorow, Hermann Hesse, and E. B. White (he of Strunk & White). Here’s White, in a letter from 1963:
Tell Johnny to read Santayana for a little while, it will improve his sentence structure.
No one could reasonably find fault with this comma splice. Its informality is obvious, and the sentence style is easy and plain.
Comma splices are especially popular with children, who tend to use lots of them in long rambling sentences. If you use comma splices in fiction or informal writing and you know what you’re doing, they should be fine. If you use them in student papers, official reports and the like, they may not. You can avoid them by using the techniques shown above.
For lots more discussion, and many examples of comma splices from literature, see my follow-up post “Oh, the Splices You’ll See!“