Colons show equivalence or explanation; semi-colons can replace a full stop to show extra closeness of two statements
Colons and semi-colons are similar in both what is written before them and what comes after. The words before a colon and a semi-colon are similar in that they will usually make a possible sentence by themselves; in other words, if you left out the colon/semi-colon plus all of the words after it, the sentence would still sound complete (for exact details of what a complete sentence is, see 30. When to Write a Full Stop). Check that there is a possible sentence before the colon/semi-colon in the following examples:
(a) (COLON) South America has a promising future: it will become very rich.
(b) (SEMI-COLON) Spanish and Portuguese are both spoken widely in South America; English and French are the main European languages spoken in Africa.
Some writers would also use a colon – but not a semi-colon – after a particular kind of wording that cannot be a sentence by itself, like this:
(c) ?The two main languages of South America are: Spanish and Portuguese.
Sentences of this kind typically have a verb immediately before the colon, commonly BE (cf. are above) or an equivalent like COMPRISE (see 162. The Language of Classification). However, this is a disputed usage. I would suggest that the only time when a colon should be used directly after a verb is when the next words (after the colon) are physically separated from it – as bullet points, for example, or in a table (see 74. Sentence Lists 3: Bullet Points).
Turning to the words after the colon or semi-colon, these are similar to each other in that they add information about the words before. This added information is very like the sort of information that a new sentence can add to the sentence before it (as described in the post 18. Relations Between Sentences). It is mostly the kind of added information that dictates whether to use a colon, a semi-colon or a full stop before it.
Colons introduce a different kind of added information from that of semi-colons. They say that the added information is equivalent to the information before (or is a reason for it), while semi-colons indicate other types of link. In example (a) above, the equivalence is between promising future and become very rich.
We could express this equivalence with an “equals” sign, like this: promising future = become very rich. In fact, a colon can be thought of as a non-mathematical equals sign. Its physical appearance reflects this: its top and bottom parts (two dots) look alike, just as the top and bottom of “=” look alike.
Equivalence after a colon may or may not be in the form of a list. It is not a list in sentence (a) above, but it is in the following example:
(d) South America has two main languages: Spanish and Portuguese.
A common alternative way of writing colon sentences, either with or without a list after the colon, is with a starting there (see 161. Special Uses of “There” Sentences). Thus, sentence (a) could begin There is a promising future for … and (d) could begin There are two…. . For a wider study of list-form equivalence, see 55. Sentence Lists 2: Main-Message. For a wider study of other equivalence, see 117. Saying More Precisely What is Meant.
Semi-colons, on the other hand, can introduce many different types of added information, but never something that is equivalent. In example (b) above, the words after the semi colon make a contrast with what is said before it. Contrast is a very common type of added information after a semi-colon, but not the only type.
There is also a grammatical difference between colons and semi-colons involving the words written after them. Words after a colon may or may not contain a verb, but words after a semi-colon must contain a verb. The words after the colon in sentence (a) above contain the verb will become, whereas those after the colon in (d) have no verb. Here is another sentence with no verb after the colon, this time without a list:
(e) Alexander the Great died of a familiar illness: malaria.
SIMILARITY BETWEEN SEMI-COLONS AND FULL STOPS
Any semi-colon can be rewritten as a full stop. This is because semi-colons have a verb both before and after them, just as most full stops do. The main difference between semi-colons and full stops is that semi-colons suggest a closer link between two statements than a full stop does.
However, there is no rule for deciding whether a link is close enough for a semi-colon instead of a full stop – it is just a matter of subjective choice by the writer or speaker, similar to the kind of choice that has to be made about paragraph length (see 59. Paragraph Length) and quoting instead of paraphrasing (see 79. Grammar Problems in Quotation Writing).
SEMI-COLONS USED INSTEAD OF COMMAS
The semi-colon use described above is often called the “weak full stop” use. A completely different use is as a “strong comma” in lists, where normally commas would be found (see 50. Right and Wrong Comma Places). Semi-colons are preferred to commas in a list when the parts of the list are long and complicated, especially if they involve commas of the non-listing kind. Here is an example:
(f) South America has some very notable features: the Amazon is one of the greatest rivers in the world; the Andes are among the tallest mountains in the world; and Brazil is one of the largest countries in the world.
It would be correct to use commas instead of the semi-colons here, but semi-colons are better because of the length of the three listed points. Note also the use of the colon after features.
PRACTICE EXERCISE: CHOOSING BETWEEN COLONS & SEMI-COLONS
Each of the following sentences needs either a colon or a semi-colon (depending on whether or not an equivalence is present). The task is to decide which form of punctuation is needed each time, and where it should go (answers below).
1. There is one computer programming language underlying all others machine code.
2. Geometry has some usefulness in the building industry there are various measurements that it facilitates.
3. Trains are safer than cars and better for the environment they should be preferred whenever possible.
4. The world is much older than previously thought four billion years at least.
5. European languages are widely spoken in other continents Spanish is found right across South America.
6. A straight line has a self-evident property it is the shortest distance between two points.
7. There is a simple solution to the problem of body weight exercise should be taken on a regular basis.
8. Physical strength depends on regular exercise academic success is not possible without constant study.
1. others: (language = machine code); 2. industry: (usefulness = facilitates measurements); 3. environment; (RESULT); 4. thought: (much older = four billion years); 5. continents; (EXAMPLE); 6. property: (property = shortest distance); 7. weight: (solution = exercise); 8. exercise; (ANALOGY/SIMILARITY)