The semi-colon is a staple of formal writing because it allows the writer to join thoughts with more sophistication than can be achieved using only the period and the comma. It pulls together independent clauses like a comma, but it allows each to retain its individual strength, like a period. Essentially, the semi-colon is a cross between the two symbols.
Note that a semi-colon can only join independent clauses; it can only be used where a period could also be used. Its effect is to show when thoughts that are independent grammatically are dependent logically on one another.
The semi-colon is most effective when it joins two independent clauses for contrast or balance:
32 I am not trying to deceive you; I am merely trying to educate you.
The writer can present two ideas succinctly without having to use a coordinating conjunction such as and or but. The clauses are at once independent and connected.
Often the first clause will be explained by the second:
33 That's odd; Jim never has a second cup of coffee at home.
The independent clauses may also be joined by a conjunctive adverb (such as therefore, nevertheless, meanwhile, besides, however, otherwise or then). In this case, the semi-colon and the adverb work together to form a closer connection:
34 The classroom was full; however, I was able to watch from the doorway.
On the other hand, one should not write:
35 The classroom was full; because the teacher was popular.
"Because the teacher was popular" is a dependent clause and cannot be separated from the main clause by a semi-colon. Be aware of the difference between subordinating conjunctions like "because" and conjunctive adverbs like "however." If you use a comma to join clauses that are linked by a conjunctive adverb you will create a comma splice. In the above examples the semi-colon operates as a kind of linking period, but it has another use in which it could be described as a strong separating comma. Ordinarily, when you write a sentence with several clauses you use commas to separate them, but if the clauses contain commas themselves, the semi-colon can be used to distinguish between the clauses:
36 I went to Hawaii because I love the sand, the sun, and the surf; because my aunt, uncle, brother and sister live there; and because my favourite cities are Honolulu and Lahaina.
In this sentence the semi-colon does not join main clauses but rather keeps the subordinate clauses clear.