What Is the Difference between a Semicolon and a Colon?
Well, some might argue that there is very little difference indeed, and the careless way in which semicolons and colons are used by many authors could be used as evidence to confirm this position. Certainly, there is very little to distinguish a semicolon from a colon in a physical sense – the first looks like a point directly above a comma (;), whereas the second places the point above another point (:). However, there are important differences between the semicolon and the colon when it comes to their functions within English sentences. As these differences are subtle at times, it can occasionally be very difficult to decide when to use a semicolon and when to use a colon, especially if the goal is to produce sophisticated prose of a scholarly standard, so these notes on usage may prove helpful.

Within normal English sentences, the semicolon has three basic functions:
• To provide punctuation between two independent clauses that are not joined by a conjunction and could, were they separated by a full stop instead, form two complete sentences. For example, ‘The first trial took place this morning; the second is planned for this afternoon.’ The two clauses are usually of equal importance or emphasis, with the semicolon implying a closer relationship between the clauses than a full stop would between two sentences. This structure can contribute to a concise style by requiring less repetition of words: were my example above written as two sentences, for instance, ‘the second’ would probably become ‘The second trial.’
• To join two independent clauses into one sentence when the second clause begins with an adverb or some other transitional phrase. For example, ‘The first trial took place in the morning; however, the second is planned for the afternoon.’ A comma usually follows the adverb or transitional phrase, as it does here, but the common practice of using a comma where the semicolon is placed in this example produces the error called a comma splice and should be avoided.
• To punctuate between the items of a series when those items already contain commas or are particularly long and complex. For example, ‘Research will be conducted in the British Library, London; the Bodleian Library, Oxford; and the University Library, Cambridge.’ In such cases, semicolons clarify both the divisions and the relationships between the items in ways commas cannot.

The colon has only two basic functions in English sentences, both of them true to the idea that a colon points forward:
• To join two independent clauses in a manner similar to that of a semicolon. When a colon is used, however, the emphasis tends to lie on the second clause, so a colon is the appropriate choice when the second clause is an explanation, elaboration, illustration, description or example of what has been introduced in the first clause. For example, ‘The first trial took place in the morning: everything went smoothly and the results were excellent.’
• To provide punctuation between an independent clause and the list, series, quotation, example or similarly explanatory or demonstrative information that follows. For example, ‘The first trial involved four of the ten groups: Indoor 2, Indoor 5, Outdoor 3 and Outdoor 4.’ As this example shows, the text following a colon need not contain a verb or be able to stand on its own as a sentence or independent clause, so the colon is particularly handy for introducing all kinds of research data.

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