Em, en, and double hyphen dash are all typographic marks used in writing, but they serve different purposes.

An "em" is a unit of measurement that is used in typography to specify the width of a dash or other horizontal character. An em dash (—) is a punctuation mark that is used to indicate a pause or break in a sentence, similar to a comma, colon, or parentheses. It is named after the width of the letter "m" in the font being used, and it is usually wider than an en dash or a hyphen.

An "en" is another typographic unit of measurement that is half the width of an em. An en dash (–) is also a punctuation mark that is used to indicate a range of values or a connection between two items, such as a span of time or a relationship between two concepts. For example, "The conference will take place from June 1–5." Here, the en dash is used to indicate the range of dates that the conference will occur.

Finally, the double hyphen dash (--), also known as a hyphen-minus, is a punctuation mark that is commonly used in computer programming languages to indicate subtraction or negation. It can also be used in place of an en dash or em dash in informal writing, where the writer may not have access to a full range of typographic symbols.

In summary, em, en, and double hyphen dash are all different typographic marks with distinct uses in writing. The em dash is used to indicate a pause or break in a sentence, the en dash is used to indicate a range or connection between two items, and the double hyphen dash is used for subtraction or negation in programming languages and can also be used in place of an en dash or em dash in informal writing.
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