In this sentence the clause introduced by which does not serve to tell which of several possible audiences is meant; what audience is in question is supposed to be already known. The clause adds, parenthetically, a statement supplementing that in the main clause. The sentence is virtually a combination of two statements which might have been made independently:
The audience had at first been indifferent. It became more and more interested.
Compare the restrictive relative clause, not set off by commas, in the sentence,
The candidate who best meets these requirements will obtain the place.
Here the clause introduced by who does serve to tell which of several possible candidates is meant; the sentence cannot be split up into two independent statements.
The difference in punctuation in the two sentences following is based on the same principle:
Nether Stowey, where Coleridge wrote The Rime of the Ancient Mariner, is a few miles from Bridgewater.
The day will come when you will admit your mistake.
Nether Stowey is completely identified by its name; the statement about Coleridge is therefore supplementary and parenthetic. The day spoken of is identified only by the dependent clause, which is therefore restrictive.
Similar in principle to the enclosing of parenthetic expressions between commas is the setting off by commas of phrases or dependent clauses preceding or following the main clause of a sentence.
Partly by hard fighting, partly by diplomatic skill, they enlarged their dominions to the east, and rose to royal rank with the possession of Sicily, exchanged afterwards for Sardinia.
Other illustrations may be found in sentences quoted under Rules 4, 5, 6, 7, 16, and 18.
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