Semicolons
"It is almost always a greater pleasure to come across a semicolon than a period. The period tells you that that is that if you didn’t get all the meaning you wanted or expected, anyway you got all the writer intended to parcel out and now you have to move along. But with a semicolon there you get a pleasant little feeling of expectancy there is more to come; read on it will get clearer.”
--Lewis Thomas
Semicolons 
are relatively heavy punctuation marks; that is, a
semicolon marks a stronger pause than a comma, but a weaker pause
than a period. It may be useful to think of a semicolon as
an addition sign that joins related ideas. They are used most frequently to
signal a close relationship between clauses, even when one is an elliptical clause
Semicolons link closely related clauses
Sequential Independent Clauses
Writers can signal that two independent clauses are closely related with a semicolon.
The mountain bike is now the most popular bike in the United States; people like its versatility.
Madonna's self -promotion makes money; she continually challenges current standards.
In each sentence, the independent clause that follows the semicolon relates directly and obviously to the first independent clause. Connecting the two with the semicolon signals to the reader that the relationship is very close.
Independent clauses connected with a conjunctive adverb
A semicolon is also used to join two independent clauses when the clauses are connected by a conjunctive adverb:
I started my homework immediately after school; consequently, I finished before dinner.
Complex independent clauses joined with a coordinating conjunction
When an independent clause contains material in which internal commas might be confused with the comma before a coordinating conjunction, (e.g., a list that also contains commas or the commas separating a non-restrictive modifier) the comma which precedes the conjunction may elevated to a semicolon:
Before the invention of the automobile, people rode horses, bicycles, or streetcars for short distances; and they used horse-drawn carriages, trains, or boats for longer trips.
Because this can result in prose that strikes the reader as odd or is difficult to read, before selecting this solution, good writers consider alternative phrasing to eliminate the problem.
Elliptical clauses
An elliptical clause is an independent clause with elements left out. They follow an independent clause that contains those elements:
In fall, Auburn students enjoy cheering the football team on to victory; in winter, the basket ball team.
In the clause after the semicolon, the clause "Auburn students enjoy cheering" has been omitted. Writers might use this construction to avoid redundant phrasing.
Items in a complex series
Semicolons are used instead of commas when they separate elements that have internal commas or other punctuation. They help sort out complex lists:
The mayors from Bangor, Maine; Hartford, Connecticut; Boston, Massachusetts; Atlanta, Georgia; and Montgomery, Alabama met to discuss common problems.
Jeremy's expenses included the cost of renting lights, costumes, and sound equipment; film and film processing fees; duplication costs for scripts, contracts, and other paperwork; and money for miscellaneous props.
In his movie, Andy wanted to include allusions to Divorce: Italian Style; And the Ship Sails On; and Five Hundred Miles.
In each sentence, the elements that the writer wants to connect already contain a punctuation mark; using another comma might cause confusion. Using a semicolon to distinguish among those elements ensures clarity for the reader.
Formatting Conventions for Semicolons
Semicolons
are placed outside quotation marks and
parentheses:
They referred to each other as "Mother" and "Father"; they were a happily married couple.
She accepted the situation with apparent calmness (but was inwardly quite concerned); however, all of that changed the next day.
There
is no space between a semicolon and the
letter or symbol it follows.
A
single space follows a semi colon.
Semicolons are sentence-internal
punctuation; this means that a
word following a semicolon should begin with a lower
case letter, unless the word is a proper noun.
Semicolon Danger Zone
The comma splice
When a comma is used where a semicolon is needed, you have created a comma splice:
Andy is a good outdoorsman, he respects the wilderness.
A comma can link two independent clauses with the help of a coordinating conjunction, but only a semicolon is strong enough to join them. Replace the comma with a semicolon:
Andy is a good outdoors man; he respects the wilderness.
Semicolon used where a comma is needed
Never having been mountain biking before; Chris wasn't really sure what to expect.
A semicolon has been used to separate a modifying phrase (Never having been mountain biking before) from an independent clause. The modifying phrase needs to be linked closely to the main clause; connect these items with a comma:
Never having been mountain biking before, Chris wasn't really sure what to expect.
Semicolons usually connect independent clauses or complex phrases that contain other punctuation.
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Ryan Dye, Duan Guiyong, Anna Head, Kyong-Mi Lee, Robin Sabino, & Kathy McClelland, 2001