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.
The writer should be careful not to set off independent clauses by commas: see under Rule 5.
4. Place a comma before a conjunction introducing a co-ordinate clause.
The early records of the city have disappeared, and the story of its first years can no longer be reconstructed.
The situation is perilous, but there is still one chance of escape.
Sentences of this type, isolated from their context, may seem to be in need of rewriting. As they make complete sense when the comma is reached, the second clause has the appearance of an afterthought. Further, and is the least specific of connectives. Used between independent clauses, it indicates only that a relation exists between them without defining that relation. In the example above, the relation is that of cause and result. The two sentences might be rewritten:
As the early records of the city have disappeared, the story of its first years can no longer be reconstructed.
Although the situation is perilous, there is still one chance of escape.
Or the subordinate clauses might be replaced by phrases:
Owing to the disappearance of the early records of the city, the story of its first years can no longer be reconstructed.
In this perilous situation, there is still one chance of escape.
But a writer may err by making his sentences too uniformly compact and periodic, and an occasional loose sentence prevents the style from becoming too formal and gives the reader a certain relief. Consequently, loose sentences of the type first quoted are common in easy, unstudied writing. But a writer should be careful not to construct too many of his sentences after this pattern (see Rule 14).
Two-part sentences of which the second member is introduced by as (in the sense of because), for, or, nor, and while (in the sense of and at the same time) likewise require a comma before the conjunction.