Non-restrictive relative clauses, which are structured like the one you’re reading now, are usually set off by a comma followed by the relative pronoun which or who. Very occasionally that is used, and its rarity (and sometime ambiguity) sounds my Curious Grammar Klaxon.
A note on terminology: non-restrictive relative clauses are also called non-defining or supplementary relatives, distinct from restrictive, defining, or integrated relatives. (There’s more on this and associated “which-hunting” in my oversized that/which grammar post.)
A Student’s Introduction to English Grammar says non-restrictive that relatives are “extremely rare and really only marginally present in Standard English”. True enough, but I tend to come across at least a few a year. Here’s an oldish one in J. W. N. Sullivan’s 1927 book Beethoven:
(1) But by the end of this summer he found that his genius, that he had felt called upon to cherish and protect, was really a mighty force using him as a channel or servant.
And a recent one in a subheading in the Guardian:
In 2011 I reported examples of the elusive non-restrictive that in prose by Penelope Fitzgerald, Anne Enright, and Robert Arthur, later adding lines from Peter Habeler and Marshall McLuhan to the annals of non-restrictive that. So it’s out there, including in well-edited texts. But it can slip by unless you’re looking out for it.
The grammar is interesting because it’s not always clear-cut. Some such lines inhabit a semantic grey area, interpretable in different ways. Fitzgerald’s line (In her mouth was something bitter, that tasted like the waters of death) might present a non-restrictive that, a restrictive clause with an anomalous comma, or an elliptical clause. Examples below will further illustrate these uncertainties.
It’s worth explicitly contrasting non-restrictive that relatives with the (superficially similar) comma splices with demonstrative pronoun that – not least because literary comma splices are commoner than you might think. Take these two sentences from Claire Kilroy’s novel All Names Have Been Changed:
(2) He kept writing writers’ novels, that was the problem.
(3) I’d even heard what the man had to say, that is the class of fool I was then.
And this similar line in Robert Harris’s cryptography thriller Enigma:
(4) It was profitable, that was all that mattered to her.
I’m pretty sure 2–4 are splices with demonstrative that, equivalent to: He kept writing writers’ novels. That was the problem, etc. You could make a grammatical case for subordinate clauses (He kept writing writers’ novels, which was the problem) using relativizer that. But I’d be very surprised if this were either writer’s intention.
Other kinds of uncertainty arise with comma-that phrases. Angela Bourke’s story ‘Pinkeens’, in her collection By Salt Water, has this line:
(5) We had real fishing nets my father got us in Hector Grey’s, that we used when we went out in the car on picnics.
That is definitely a relativizer here, but what sort of clause is it in? It could be restrictive with an excrescent comma, the stop inserted for rhythm or because of the distance between fishing nets and the relative pronoun; or it could be non-restrictive but with that in which‘s usual slot: fishing nets my father got us, which we used when…
We see the same ambiguity in The Last of Squire Ennismore by Charlotte Riddell:
(6) To his left lay the purple headland; to his right, a long range of breakers, that went straight out into the Atlantic till they were lost from sight…
The close or heavy style of punctuation was more prevalent when this story was published (1888), which may suggest that the comma after breakers was included for prosody or personal style, not grammatical necessity. On the other hand, non-restrictive that used to be a lot more common than it is nowadays.
Compare the preceding examples with this one from Ira Levin’s Rosemary’s Baby:
(7) They were in one room, that had been Guy’s bachelor apartment.
I’m fairly sure this is a non-restrictive that, though taken out of context you could parse it as a splice.
Finally there is the following interesting example,
that which appears in ‘A Legend of Knockmany’ in the Joseph Jacobs’–edited Celtic Fairy Tales (1892)
(8) Oonagh, or rather Fin, lived at this time on the very tiptop of Knockmany Hill, which faces a cousin of its own called Cullamore, that rises up, half-hill, half-mountain, on the opposite side.
Style comma or grammar comma? I keep changing my mind on this one, like with the optical illusion of inverted boxes. Maybe which rises up was dispreferred because of the earlier which faces, and the use of different relative pronouns helped contrast the two hills being described. But whether or not that’s the case, it proves nothing.
Ambiguity: don’t you love it?
At You Don’t Say, John E. McIntyre reports his observations from the Baltimore Sun copy desk:
Over the past few years, I’ve come across a handful of non-restrictive thats nearly every week at The Sun, and I am puzzled at their appearance. For the moment, I can only offer a tentative surmise, that the that/which fetish has put it in the minds of uncertain writers that there is something tricky about which, so that is safer.