840. Use a dash (—) when the thought is abruptly broken off.
Just then the telephone rang—I wish I could remember who was calling.
841. Use dashes rather than parentheses to inclose informal parenthetical material.
He offered me—imagine it—twenty dollars a week.
842. Use a dash before a summarizing expression.
After the wheat is all in, after the horses are put up, and after the chores have been done—then you may use the car.
843. Never use a dash where a period should be used.
Wrong. Come tomorrow—everybody will be here then—
Right. Come tomorrow. Everybody will be here then.
844. Never use a dash where a comma should be used.