When isolating thoughts with commas, dashes and semicolons, people tend to mix and match these marks of punctuation for some odd reason. As if they don’t want to leave any of them out of a particular sentence.

Let’s get one thing straight: it is absolutely correct to use more than one type of punctuation in a sentence. But if you want to isolate a thought, you need to use the same punctuation mark at the beginning and end. Unless a period is going to finish off everything.

For example, if you include an em dash (—) to separate a thought within a sentence, cap it off with another em dash. Don’t suddenly have a change of heart and use a comma instead. You’re probably thinking, “Why on earth would someone do that?” It’s more common than you’d think, hence the need to draw attention to it.

Let’s look at this scenario in the following example:

Doug was worried about his cat Felix — albeit slightly — while he was in surgery.

The “albeit slightly” being isolated in this case could have been separated by commas as well. But starting with an em dash and finishing with a comma is not okay. It looks sloppy and careless. And it’s wrong.

Semicolons (;) should only be used in one instance, separating two thoughts that are related to each other. Unless you’re listing something that requires deep explanation. You can read more on that here.

For the most part, commas and dashes are interchangeable. Just remember to keep it consistent within the same thought. Fewer editors around the world will be cringing. And we’d like to thank you in advance for that.