There are two main types of dashes — em rules and en rules. An em rule is used to indicate the punctuation mark we know as the dash. The en rule is used to indicate an association between words and to link spans of figures.
Em dash
The em rule has three main uses:
- to signify an abrupt change
- to introduce an explanation or expand on a point
- in place of brackets.
...
- The main reason for exercising is to lose weight — but this is not the only reason.
- John was the best in the state — he had won the state championships before.
- He typed the manuscript — the one I submitted — without errors.
En dash
The unspaced en rule is used to:
- show spans of figures, time and distance
- show an association between words that are separate identities.
...
- say between 1975 and 1999, not between 1975–1999
- say from 1975 to 1999, not from 1975–1999.