Although two or more main clauses can be combined
into one sentence by using ", and," ", or," or ", but," three punctuation
marks can also be used not only to combine the clauses, but also to direct
readers to see specific logical relationships between the ideas expressed
in the clauses.
Colons and Dashes to Indicate
A colon or a dash can be used to indicate a
"general/specific" relationship between the ideas in two main clauses:
The weather was nice -- it was sunny with
a soft wind.
The payment is late: it was due two weeks
In these examples, the first main clause makes a general statement, and
the second provides more specific details.
Semicolons to Emphasize Contrasting
Consider the following two sentences:
He went swimming. She did the dishes.
In effect, they simply state two facts. We can combine them with ", and"
and a small "s," but they will still simply state two facts:
He went swimming, and she did the dishes.
There is, however, another way of combining the two, and it changes the
meaning. When a semicolon is used between two main clauses, it suggests
that the clauses embody contrasting ideas. Thus, we could write:
He went swimming;
she did the dishes.
The semicolon invites the reader to think about the differences between
the two main clauses, and, in this case, a little thought suggests that
the underlying contrast here is that he is having fun, but she was stuck
working in the kitchen.