Dashes, like table forks, come in different sizes, and there is a proper use for each. The handiest member of the dash family is the one-em dash. Think of it as the dinner fork, the one on the inside, which you use for your main dish. ... We who grew up using typewriters learned to type two hyphens, with or without space on either side, to form a dash. Most word-processing programs automatically fuse double hyphens into a solid dash, when you get to the end of the word following it — that is, the computer automatically compensates for old-fashioned habits. There is also a one-en dash, the width of a capital N. That would be your salad fork. Some clever writers type two one-ens instead of two hyphens to form the long dash, and it looks good, but it's brittle if it falls on a line break, it snaps in half. ...
With Emily Dickinson at the table, my simplistic division of dashes into table forks and salad forks falls apart. She used dashes for everything, and sometimes for two things at once. If a different size and style of fork were assigned to each of her various dashes, the table setting would require not just dessert forks and fondue forks and those tiny forks used for teasing out snails but also tuning forks and pitchforks.
by Mary Norris