The Welsh artist and poet David Jones called his war memoir ‘In Parenthesis’ to express the in-between-ness of the First World War; it caused a rupture between the past and present, between peoples, and between cultures. But the war also had its own existence, separated off from the rest of history before and after. He recalls how glad people thought they were, in 1918, ‘to step outside its brackets’.
And that, of course, is what parentheses are, grammatically speaking. Not just the punctuation mark, but the explanatory or additional wording that goes inside the brackets.
When it comes to showing the relative significance of ideas in a sentence, there is in fact a hierarchy of punctuation marks that can be deployed: not just brackets, but also dashes, or, simply, commas.
So, which kind of punctuation should you choose? That’s partly determined by the degree of relevance of the parenthesis to the main clause. Consider the following sentences:
The man, who made his fortune from cookies, was an expert baker.
The man — who made his fortune from cookies — started out poor.
The man (who made his fortune from cookies) was South African.
In sentence 1, we understand that there is a strong relationship between his skill as a baker and the fact that he made money from cookies. We use commas to show that there is no real separation of detail.
In sentence 2, the fact that he was originally poor is stressed, but the source of his fortune is not so important — it just happened to be cookies.
By the time we reach sentence 3, how he made his fortune is not at all relevant, and the brackets allow us to safely remove the information about the cookies from the main thrust of the sentence.
Learning to choose which degree of separation takes practice, but one rule is absolutely sure. In all three cases, the sentence must still read correctly if we take out the parenthesis. This is a crucial test for proofreaders’ understanding of punctuation. So something like this:
4. The book (they claimed) had been stolen was found.
cannot be correct. Omitting the words in brackets gives the ungrammatical ‘The book had been stolen was found.’ But place the brackets more logically around ‘they claimed had been stolen’ and it works fine.