Place a Comma before and or but introducing an independent 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 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.
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.
