The main differences between em, en, and the double hyphen dash are their lengths and usage:

- Em dash (—): The em dash is the longest of the three and is typically the width of the letter 'M' in the font being used. It is used to set off parenthetical information, indicate a pause, or to emphasize or clarify a specific part of a sentence. Example: "The secret ingredient—chocolate—made the cake irresistible."
- En dash (–): The en dash is shorter than the em dash, approximately the width of the letter 'N' in the font being used. It is primarily used to show ranges, such as numbers, dates, or time. Example: "The meeting will be held from 2–4 p.m."
- Double hyphen (--): In the past, when using a typewriter or when certain character sets were not available, the double hyphen was used as a substitute for the em dash. Nowadays, it is less common due to the availability of em and en dashes in most word processing software and fonts. Example (em dash substitute): "She decided to take a break from work--she needed a vacation."

In summary, the em dash is used for emphasis and clarification, the en dash is used for ranges, and the double hyphen is an outdated substitute for the em dash.
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