Place a Comma before and or but introducing an independent clause

ELEMENTARY RULES OF USAGE

 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 after-thought. 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.

Enclose Parenthetic Expressions Between Commas

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.

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.

If a dependent clause, or an introductory phrase requiring to

be set off by a comma, precedes the second independent clause,

no comma is needed after the conjunction.

The situation is perilous, but if we are prepared to act

promptly, there is still one chance of escape.

For two-part sentences connected by an adverb, see the next

section.

Source: The Elements of Style by William Strunk, Jr.

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