Go not too far into any grammar of English and you’ll come upon a chapter on the relative pronoun. This device—who, whose, whom, which, and that are the basic forms—gives us a way to refer to something again in a sentence without using the same noun over and over: Her new film, which opened last night, is outstanding. The relative pronoun which refers to the noun film, and with it we can avoid the simplicities of a compound sentence: Her new film opened last night and it is outstanding.
The relative pronoun which, though, has a near cousin called the relative adjective. Like its relative, this relative refers to a noun in the immediately previous clause, but goes on in its own individuality to modify a noun in its own clause: His proof depended on three receipts, which vital evidence he had lost. Here, the word which is bridging an independent clause (proof depended) with a subordinate clause (evidence he had lost), and this is exactly what grammatical relatives are meant to do. But as an adjective—and it is an adjective because it is modifying the noun evidence—the word which has the further effect of emphasizing the meaning that attends on its antecedent, the three receipts. With that relative adjective modifying an already modified noun (which vital evidence), the writer is taking no chances that we readers miss the point: those three receipts were more than incidental; they constituted the core evidence the unfortunate defendant needed to prove his case. And so the written scene is enriched.
Here’s another example, this one a bit longer: His three-year tenure as CEO had been marred by scandal, and the board of directors had finally had enough. They confronted him at the fall meeting, at which time, surprisingly, he offered his resignation without objection. Once again the word which, as an adjective, is modifying the noun time in the subordinate clause, while also bringing our attention, subtly and surreptitiously, back to the fall meeting, the originating antecedent of the relative adjective. This hearkening back to an idea, simultaneously referring to it by way of a synonym, is what marks this a richer, plusher sentence. The writer wants us to understand that there and then the embattled CEO decided to get out without a fight. That was a surprise, no doubt, and the writer did not want to risk our misunderstanding the unusual turn of events. Hence the more involved construction to match the involved corporate intrigue.
It is common to object to less common constructions like these by suggesting they be reduced to simpler forms. Why not just this: His proof depended on three receipts, and he had lost them; or this: They confronted him at the fall meeting, and he offered his resignation without objection: No one revision can be judged the better choice without understanding more of the context in which the sentence in question appears, but we can safely assert that simplicity for the sake of simplicity is not the right direction. Simple is good, but simplistic is not, and we can too easily slip from one to the other across a paragraph, or even an entire document, when our sentences show little variety. Form must conform to its content, which means we must do more than communicate the bare simple meaning; we must also allude or intimate or suggest the other forces that surround the ideas we’re writing about. We may wish life were simple, but it and we in it are not. And whether we call life complicated or dynamic, our thoughts about it are all the richer and our writing the more robust when we strike with sentences which in their own structure—sometimes simple, sometimes complex—mirror the whirling world in which they, and we, arise.