There are basically three dashes (four if you want to get really picky) that are in common use in written language. They each serve very specific purposes and should not be used interchangeably. Like all forms of punctuation, they clarify further the authors intent by their correct use. By comparison, when they are not used correctly they work counter to the meaning of the words that accompany them. The incorrect usage of dashes is particularly widespread primarily as a result of the ease of desktop publishing. The fault usually lies in the fact that with the exception of the hyphen, the other dashes are classified as a “special characters” and do not appear on any of the keys of a computer keyboard. Therefore, to create one the writer or designer must know not only the distinction between all three but also how to create each one.
Here they are in all their glory
Should be used only to hyphenate words (made directly from keyboard hyphen key).
EN dash –
Should only be used between phrases that indicate durations of time such as 1:00 – 2:00, Monday – Friday, 1919–2019, etc. (made by pressing both option and hyphen keys simultaneously).
Basically, any other instance requiring a dash that is not a hyphenated word nor separating durations of time. Common examples include the dash proceeding an author’s or poet’s name in lieu of the word “by” or in a compound sentence where there are multiple clauses. Those clauses might alternatively be separated by parentheses or commas. The choice of which is usually determined by where you want to place emphasis. The clause enclosed by commas typically has the same emphasis as the other point within that sentence. Separation by use of parenthesis gives a bit more. Separation by EM dash gives the most emphasis to that part of the sentence. (make one by pressing the shift, option, and hyphen keys simultaneously).
For all of the very technical precision and subtle clarity that correct use of these marks impart to the written word, spacing of them, by comparison is rather arbitrary. No two designers necessarily space these the same. There is no hard and fast rule to aid you. What you must do instead is apply the same thinking that you do when kerning. Your goal is optical evenness. Avoid the creation of awkward “hot spots” because of the particular letter that immediately proceeds and follows the dash. You must adjust this space, before and after the dash, so that they appear equal in length. Whatever that result is you must try to maintain it throughout the rest of the document on every additional use of that form of dash. If it is a “close” spacing on one it must remain that same closeness on all the other ones. The same is true if you determined the spacing to be a bit wider. Then it must remain that version for the rest.