When it comes to separating thoughts and clauses, this tends to be quite a complex distinction to make. Especially when you involve colons and commas as well. Oh, the joys of the English language! For the most part, a semi colon and an em dash are interchangeable — used to separate an afterthought (much like this one).

Just in case you wanted to know the full story, here is a more in-depth breakdown of each:

The Semi Colon ( ; )

Commonly confused in its use (especially with the colon), the simplest way to describe the proper use of a semi colon is as a combination of a comma and a colon. Use a semi colon when you want to separate a thought that is being introduced by the preceding one. Just keep in mind that both these thoughts work together to give you the ultimate point of the sentence. A colon introduces something more direct, like a list.

This is what we’re telling you: colons are more direct.

The message has been received; thank you.

See how the colon gives you two separate clauses while the semi colon divides the same thought into two parts?

Another good use for a semi colon is when you have a list within a list. In this case, you would use the semi colon to separate bigger clauses and a comma for the points within those clauses, as follows:

I visited France, Spain, and Italy in Europe; New York, Toronto, and Chicago in North America; and Thailand, Vietnam, and Japan in Asia.

The Em Dash ( — )

The em (M) dash (commonly confused with the shorter en (N) dash) is the longest of the dashes, named after its shared width of the letter M, as the en dash is named after its [shorter] shared width of the letter N.

An em dash is used to separate a thought for effect, or to separate a thought or point amid other clauses in a sentence. If a sentence is already full of commas and semi colons, a dash can act as an isolator. Although, if you get to the point where you’re using all three, it’s probably best to break it up into multiple sentences.

An em dash is effective in separating the final thought of a sentence — as a punch line of sorts. See? Or if you want to make a dramatic point — like this.