A semi-colon can be used to separate two main clauses in a sentence. Think of a main clause as being a sentence in its own right. For example, below, we could replace the semi-colon with a full-stop to form two separate sentences and not a single grammar crime would be committed:
Doug went to the shop to buy a newspaper; he eventually chose a copy of the Daily Mail.
Doug went to the shop to buy a newspaper. He eventually chose a copy of the Daily Mail.
So, why bother using a semi-colon if a full-stop will do the job? Well, a judiciously placed semi-colon can help to imply a subtle link, or relationship, beyond what is explicitly stated by the two separated main clauses. Here’s an example from the novel A Place of Greater Safety, by Hilary Mantel:
‘There was the drawing room life; then there was this other life. She thought it was best not to ask questions about it.’
The novel is set in revolutionary France and tracks the fortunes of, amongst others, rakish political journalist Camille Desmoulins . In the quote above, Annette Duplessis, who is an outwardly respectable and dutiful mother of two, is reflecting on the unwholesome double-life that Camille appears to lead. Here, the semi-colon serves to highlight this duality (one sentence, two parts), but it also hints at the sense of discomfort and uncertainty she feels at being in possession of this knowledge.
Here’s one more example of how a semi-colon can be used to split two main clauses, taken this time from The Go-Between, by L. P. Hartley:
‘One of these confidences was our respective addresses; he told me his home was called Brandham Hall and I told him mine was called Court Place, and of the two he was more impressed’.
The novel is about a young boy called Leo who spends the summer of 1900 staying with Marcus, his much more affluent friend from school. Marcus is a snob and, at least in part, invites Leo to stay at Brandham Hall because he’s under the misconception that Court Place is a very grand stately home. In the quote above, where Leo recalls a conversation on this subject, the semi-colon conveys a pleasing sense of immediacy and continuity: we are given the impression of the conversation having recently taken place and of information having only just been exchanged, as if part of a transaction.
And so, why does all this matter? It doesn’t, really. Mostly, a full-stop will work best when you want to convey information precisely. However, opportunities do occasionally arise and, when they do, it’s worth keeping these words from Hilary Mantel (once again), in mind:
‘There’s nothing in this breathing world so gratifying as an artfully placed semi-colon.’
True dat. Finally then, if you would like to test your knowledge of semi-colons (maybe this weekend or even tonight), this is a decent resource:
TL;DR: a semi-colon can help to imply a subtle link beyond what is explicitly stated by two main clauses.