Few punctuation marks are as misunderstood as the semi-colon.
A semi-colon does not just indicate a longer pause than a comma. Its main purpose is to connect independent clauses. Occasionally you can also use a semi-colon in a list.
Most of the time, a semi-colon is used to connect two independent clauses that are related in theme and content:
Buy a van if you have a family; buy a truck if you want a family.
Yesterday she upped her exercise routine; today she’s laid up in bed.
I am learning Old French; I want to read The Song of Roland in the original.
A hummingbird flashed by the window; it briefly hovered by the fuchsia and then flew away.
As these examples show, often the semi-colon is used for juxtaposition or to connect two ideas that form a whole. Sometimes you could substitute a dash, though the effect is more casual, with the result that the connection between the clauses is less clear.
Use the semi-colon sparingly. Having more than one in a single paragraph can slow down your prose and may appear ostentatious. In addition, even if a semi-colon is grammatically correct, it may not be your best option. Take the following examples:
Oswald Mosely was the leader of the British Union of Fascists; in 1931, he traveled to Italy to study Mussolini’s brand of fascism.
I enjoyed listening to Michael Nyman’s score for the movie The Piano; Nyman’s compositions remind me of the piano music of Ludovico Einaudi.
In each sentence the two clauses have some bearing on each other, but they do not strictly need to be connected by a semi-colon. You could use a period instead.
The next rule is merely a minor variation on the basic use of a semi-colon. When you connect two independent clauses with a semi-colon, you can add a transitional expression to the beginning of the second clause:
A group of librarians is called “a shush of librarians”; by contrast, a gathering of language experts is termed “a babble of linguists.”
I’m studying to become a math teacher; however, I really enjoy taking English classes.
The transitional expression is usually a conjunctive adverb.
Normally a semi-colon should not be followed by a coordinating conjunction (and, but, nor, for, or, so, yet). Avoid the following:
In 2015, more people than ever attempted to climb Mount Everest; but no one reached the summit.
In such cases a comma will do the job. Still, there is one exception. Occasionally, you can us a semi-colon before a coordinating conjunction when both clauses are quite long and involved (i.e., they contain internal punctuation):
This is a tortuous sentence, which twists and winds, and can easily confuse the reader; but a person of wit and intellect, attuned to the vagaries of language, will appreciate its clever use of a semi-colon.
If you’re not sure, just stick with a comma.
The last use of a semi-colon is quite different from the others. It is also very rare. The rule is that you are allowed to put a semi-colon between the items of a list if the items themselves already contain punctuation:
In the 1890s, Canada had four Prime-ministers in quick succession: John Abbott, who led from the senate; John Thompson, the first Catholic Prime Minister; Mackenzie Bowell, who had to deal with the Manitoba Schools Question; and Charles Tupper, a father of Confederation.
You will notice that each item in this list already has a comma (e.g., John Abbott, who led from the senate), so using semi-colons provides some clarity and organization. If there is no internal punctuation, you can use commas instead.