Unlike the en dash which is often confused for a hyphen and substituted as such, the em dash play a unique role in English writing. However, before we get into that, let’s talk about what the em dash is and where it’s name comes from.
This is the em dash: —
This is an en dash: –
This is a hyphen: -
See the difference? The em dash is the longest dash of them all. It’s called the em dash because it’s as long as the letter “M.” The en dash, on the other hand, is only as long as the letter “N” which is shorter than “M.” Now that we got that out of the way, let’s focus on what the em dash is used for.
Em dashes help to set apart phrases or sentences within a sentence that would otherwise be hard to read. Take the following sentence for example.
The rock band Led Zeppelin, often referred to as the fathers of heavy metal with multiple platinum albums in their discography, including their big hit “Stairway to Heaven,” recently released a re-edit of an old album, “Physical Grafitti.”
As you can see, the sentence above is a little clunky and hard to read. The reason for this is the parenthetical clause through the use of commas. When using many commas within a sentence, you should consider using an em dash instead. Take a look.
The rock band Led Zeppelin—often referred to as the fathers of heavy metal with multiple platinum albums in their discography, including their big hit “Stairway to Heaven”—recently released a re-edit of an old album, “Physical Grafitti.”
See how much easier it is to read now? The em dashes help to separate the the parenthetical clause within the sentence which make it a lot easier for the reader to digest.
As a basic rule of thumb, if you’re going to make a parenthetical statement that has commas, it’s always best to set the clause apart through the use of em dashes. They emphasize the temporary departure from the main sentence more than commas, but they’re not as obtrusive as parentheses.
The real question is whether or not you want to put a space before and after the em dash. I prefer not to, but there’s no consensus on the matter so feel free to do whatever feels right for you!