"He went swimming. She did the dishes." Those two sentences are what started me working on the problems of teaching grammar. I was, at the time, a graduate assistant at Cornell, and I was trying to explain to my students how a semicolon can be used to emphasize the contrast between ideas that are expressed in two main clauses. After the semester was over, one student told me what my problem was -- few, if any of the students understood what a "clause" is. That realization led to the KISS Approach to teaching grammar. The explanations here are intended for my students in Freshman Composition, students who, by the time they use this page, should be fairly familiar with recognizing clauses.

     Main clauses can be joined by a comma plus "and," "or," or "but," but they can also be joined, often more effectively, by a semicolon, a colon, or a dash. 
     Many handbooks suggest that the semicolon be used to emphasize contrast, and many good writers follow this suggestion. As a result, good readers often look for an underlying contrast in ideas separated by a semicolon. Whereas "He went swimming. She did the dishes." simply states two facts, a semicolon between them would invite the reader to consider the differences in what he and she are doing: "He went swimming; she did the dishes." If we stop to think about this difference, it doesn't take too long to realize that he is having fun, whereas she is stuck in the kitchen. To remember this function of the semicolon, simply note that it is composed of two similar, but different marks -- a period over a comma. In essence, the semicolon indicates that the two sentences are in some way the same (In the example, both he and she are doing something.), but that there is also an implied difference.¹
     In contrast to the semicolon, colons and dashes are best used to combine two main clauses when the second clause amplifies the first, often by giving a more detailed version, or an example, of the first.

The weather was bad -- it was cold and windy.
She worked hard: she raked the leaves and washed the windows.

A foot-hold trap is exactly what it sound like -- a trap that captures and holds the animal by the foot.

A foot-hold trap is exactly what it sound like; a trap that captures and holds the animal by the foot.

Exercise 1
Exercise 2 (Revisions)

     The average professional writes about twenty words per main clause, which means that the average professional reader is accustomed to processing sentences within that range. Some students, however, average 25 words per main clause. Such complexity may overwhelm your readers' (including your instructors') processing ability. [In other words, your paper may get a low grade not because of what you wrote, but because of the complex way in which you wrote it.] The following exercises provide practice in decombining long main clauses into shorter ones.

Exercise 1

1. Some students spread semicolons throughout their papers as if the punctuation marks were salt and pepper. Semicolons have only two functions: 1) to separate main clauses (preferably when they present contrasting ideas), and 2) to separate items in a list (such as addresses) when the items in the list themselves contain commas. Inserting semicolons for any other reason will cause problems for readers because the semicolon indicates a dump to long-term memory. (See the psycholinguistic model of how the brain processes language.)

2. Some grammarians don't agree that main ideas usually appear in the main S / V / C pattern, but these same grammarians rarely study sentences in context, nor do they teach their students to do so. Once you can identify subordinate clauses, you can decide this question for yourself.