Comma splices are probably the punctuation mistake I see most often, and it's frequently in writing by people who otherwise know why punctuation matters, and use it very well. And if you're thinking, "Real readers don't care about these rules!", then consider what I said in this post.
Few of us have absolute pitch, only some of us will consciously notice that a choir isn't in tune, but many, many more will simply not feel/get/enjoy/be-swept-away by the music as we would be if it were in tune.
As with singing out-of-tune, when you commit a bad comma-splice you won't be having the effect on your audience that you're trying for.
What is a comma splice?
The grammatically complete* sentence is the basic unit of written language. Commas are used to separate out different units within a sentence to make its meaning clearer, which is usually (but not always) the same job as evoking where a speaker's voice would lift slightly. Full stops (= AmEng periods), on the other hand, bring the sentence to an end by evoking a more substantial pause, ready to launch into the new one.
All of these sentences contain a comma splice:
- She sleeps soundly, he lies awake and reads.
- She bought nearly all the vegetables, it wasn't by accident she forgot the potatoes.
- He was hit by her till he cried, it was the only time she hit him.
- They parked the car and locked it, they went to the park for a stroll.
- Suddenly, they will horrify the guests, however, brandishing a carving knife won't be scary enough.
And if you don't believe me, read them aloud. Indeed, reading aloud is a brilliant, intuitive way to pick them up.
Why do comma splices matter?
When two grammatically complete sentences, which really need a full stop between them, are separated by a mere comma, the grammar and syntax of the first sentence lead to the finish, but the comma isn't a firm enough stop. So the first sentence seems to tumble on, over the break, and land on the beginning of the second – which then doesn't get to launch properly. The paragraph as a whole feels sloppy and incoherent, as reading aloud will reveal, even if your eye didn't spot it.
How bad the effect is will depend very much on the sentences involved: of that lot above, in No.1 the comma splice is hardly a problem at all, but No.4 is barely comprehensible. And, generally speaking, if your reader has to re-read something to make basic sense of it then as the writer you have failed.
What's more, because this is a basic failure which writers, agents, teachers and editors see a lot of, our ears are sensitised to comma splices. At the very least we will sigh that they'll all need unscrambling; at the worst, we'll think this is a writer who can't be bothered to learn how punctuation works. Luckily, though, they're easy to get rid of.
When I've spotted a comma splice, what can I do about it?
Have a look at your two sentences.
a) Try a full stop: if they really are grammatically complete sentences, that will always solve the problem:
- Suddenly, they will horrify the guests. However, brandishing a carving knife won't be scary enough.
However, comma splices usually get in there precisely because the writer instinctively knew that a full stop broke the connection between the sentences too absolutely for what they're trying to say. In which case,
b) Try a semi-colon to separate two independent but closely connected sentences:
- She sleeps soundly; he lies awake and reads.
c) Try a colon to separate the main sentence from the one which provides further information about it:
- She bought nearly all the vegetables: it wasn't by accident she forgot the potatoes.
d) Try a comma with a co-ordinating conjunction (= and, or, but, while or yet):
- He was hit by her till he cried, but it was the only time she hit him.
Note that different conjunctions will have different effects on the overall meaning.
e) Try a mild re-jig of the sentence
- They parked the car and locked it, then went to the park for a stroll.
p.s. then is technically not a coordinating conjunction and ought therefore to have an and before it, but that makes two ands rather clunkily close together. This way is a perfectly clear and much-used way of phrasing such things, so why not?
* You do know a grammatically complete sentence when you read one: one with a main verb (also called a finite verb) in it:
- I sleep. He lies awake, reading.
- She hit him. It was the only time she hit him.
- They parked the car and locked it. They went to the park for a stroll
- Suddenly, they will horrify the guests by brandishing a carving knife.
And you also know a sentence fragment - an incomplete sentence, lacking a main verb - when you read one:
- He awake, reading
- The only time she hit him
- Parked the car
- Suddenly brandishing a carving knife