Some writers use a dash when they should use a semi-colon, and others do the opposite. There are actually several uses for each. You can Google around to find out all the details, but these two ideas are very useful and easy to remember:
1.) Connecting independent clauses with a semi-colon
A semi-colon is used the way a period is used: to connect independent clauses.
"Jose won the contest. Everyone expected him to win."
"Jose won the contest; everyone expected him to win."
**An independent clause is a group of words that could be a complete sentence if they wanted to be, even though they might choose to be part of a longer sentence. Do not use a dash to connect independent clauses. Use a semi-colon!
2.) A dash works like a comma, but a really big one. A dash is very useful for managing long sentences, especially ones that already contain a few commas -- this sentence being an example -- because using a set of 2 dashes can help you to avoid using lots of confusing commas. So, a dash is like a really big comma that takes itself too seriously.
Another example:
Jose won the contest, an impressive accomplishment, and although he felt good about the victory he could not help thinking about the disappointment his opponent must have felt.
This could be written with dashes:
Jose won the contest -- an impressive accomplishment -- and although he felt good about the victory he could not help thinking about the disappointment his opponent must have felt.
If the sentence needed to be longer, the dashes would be extra helpful for keeping the sentence clear:
Jose won the contest, an impressive accomplishment, and although he felt good about the victory he could not help thinking about the disappointment his opponent must have felt -- because this contest occurs only once every four years.
In this last example, you can see that the dash works just like a comma. It is useful, though, for helping the reader to compartmentalize the many parts of the sentence and think about it in an organized way. Part of being a good writer is the ability to accommodate the reader's quirky attention.
NOTE: This is not a complete explanation for the uses of the dash and semi-colon. There are other uses; for example, a semi-colon is often used to separate items on a list. In this blog, we just wanted to share the insights above and leave you with this easy-to-remember concept:
"A dash works like a comma that takes itself too seriously, and a semi-colon works like a period that does not take itself seriously enough."
