Semicolon usage

As you mentioned, the semicolon is most commonly used to join two closely related independent. The semicolon has two other less common uses.

First, lets start with the idea of a series. Items in a series are normally separated by comma. For instance: Columbus sailed across the Atlantic Ocean with Nina, Pinta, and Santa Maria. You see how the comma is used to separate the names of the three ships.

Now, what happens when you have a complex series? A complex series occurs when an item contains an internal comma. For instance:

John, the oldest brother

Paul, the short brother

and Peter, about whom the less said is better

How do we assemble these into a sentence without all of the commas becoming confusing? Instead of using a comma to separate each item, we use a semicolon: Also traveling together were John, the oldest brother; Paul, the short brother; and Peter, about whom the less said is better.

The other use goes back to joining independent clauses. Certain adverbs -- however, thus, hence, indeed, accordingly, besides, and therefore -- can also function as conjunctions that join independent clauses. When any of these are used, we always use the semicolon (and never the comma) to join the clauses.  Independent clause A; however, independent clause B.

I hope this helps.