Colons and semi-colons (along with their cousin the comma) organize, separate and control individual elements of a sentence. They have not always been included in the world of punctuation; the semi-colon did not appear in common usage until the 1400s for example. Early sentence structure relied more on end punctuation, such as periods and question marks, to separate independent thoughts. The advent of both the colon and the semi-colon allowed writers to form compound-complex sentences, which gave the written language much more variation and interest for readers.
The colon looks like two periods stacked vertically (:). It may help to keep the period in mind as the rules of colon usage are studied. Much like the period, colons create a definite stop in the flow of a sentence. The most common use of this stopping power is to create anticipation for a long list of elements. An independent phrase, one which could stand on its own, often introduces such a list: "The following articles should be read by all wiseGEEK visitors: Euphemism, Pseudonym, Haiku and Palindrome." In this sentence, the colon is used to set up a list in the reader's mind. The opening phrase is independent and strongly implies that a list is to follow. The colon is used to separate the set-up from the list itself. Individual elements of that list are separated with commas. If the list were even more complex, semi-colons may also be used to separate the individual elements: "Speakers at the convention were a cross-section of today's brightest minds: Dr. S. Jones, lead neurosurgeon; S. Smith, mission coordinator; Reverend J. Harris, parish spokesman; and J. Saunders, public relations officer." Colons set up lists, but do not separate individual items in those lists. The colon should not be used in situations where the set-up is not independent: "The students who won the spelling bee were J. Smith, J. Doe and T. Johnson." In this case, no colon would be necessary.
Another use of the colon involves quotations. If the quotation is lengthy, a colon is often use to set it apart from the rest of the sentence. "Mrs. Jones reminded the students of the words of Abraham Lincoln: "Four score and seven years ago, our forefathers brought forth upon this land a new nation, conceived in liberty and dedicated to the idea that all men are created equal." Colons should not be used to separate short quotations, however. "Forrest looked at the boy and said, "Stupid is as stupid does." Colons are also used to separate the dialogue in a play or court transcription:
CATHERINE: I heard every word you said back there, Jeff.
JEFF: (angrily) And you still let me believe you weren't home?
CATHERINE: I didn't think you cared.
Colons can also be used to separate hours from minutes when writing times in a sentence: "She told me to show up at 8:15 in the morning, but I didn't see her until 8:45." Some style manuals also suggest that colons can be used to connect two related sentences. Instead of writing "The plane is always late. The pilot never leaves on time.", a colon could be used to create a compound-complex sentence: "The plane is always late: the pilot never leaves on time." This use of the colon is not common, but it does fix the common grammatical error called a comma splice. Some writers improperly connect two sentences with a comma, which can often create a very awkward combination. Using a colon when the two sentences have a definite relationship is an acceptable fix.
Semi-colons are also used to correct comma splices. A semi-colon encourages the reader to form a relationship between two independent clauses: "The snow covered the walkways and entrances; officials called off school for the day." In this case, the semi-colon implies a cause and effect relationship between the realities of the weather and the actions of the officials. The same sentence could contain a transition as well: "The snow covered the walkways and entrances; therefore, the officials called off school for the day." Semi-colons do not create the same level of anticipation as colons, but they do create a stronger pause than commas.
Commas are often used to separate elements of dependent clauses, but semi-colons only connect independent elements. Writers sometimes use dashes (-) in place of semi-colons to add a related thought, but semi-colons should be the default punctuation when connecting sentences with definite relationships. Beginning writers will often create simple sentences in order to avoid using semi-colons, but practice and understanding of the principles behind punctuation should reduce these fears.