Post by Pamela
Joining them serves to connect their common idea. The text before and after
deals with a somewhat separate matter.
The usual guidelines recommend to join main clauses with a semicolon.
When a comma is used instead, that's an error called a "comma splice".
When nothing at all is used, that's an error called "run-on sentence"
or "fused sentence".
When a coordinating conjunction is used to join the two main clauses,
then a comma is used instead of the semicolon.
Coordinating conjunctions are FANBOYS:
F = for; A = and; N = nor; B = but; O = or; Y = yet; S = so
(I believe that "while" sometimes is used as a coordinating
conjunction, which is disapproved by Fowler's.)
When the second sentence is subordinate, no punctuation is required.
(Sometimes, special rules allow the omission of some
punctuation in the case of very small clauses.)
So, one writes, for example,
He wore a lovely hat. It was his only defense.
He wore a lovely hat; it was his only defense.
He wore a lovely hat, for it was his only defense.
He wore a lovely hat because it was his only defense.