Short sentences are great. They’re generally easier to read. However, sometimes a side comment is needed to drive a point home — you know what we mean? Luckily, there are not one, not two, but three different ways to add an extra thought to your sentence. Commas, parentheses (or brackets), and em dashes can add clarity, helpful context, and poetry of prose. Which one should a writer use, and when?
How Formal Is the Writing?
The formality of the situation is the main deciding factor for stylistic (i.e. not grammatically required) punctuation. Commas are the most formal and the least intrusive. Em dashes are the least formal, but also the most emphatic — they tend to call the most attention to the aside. Parentheses are somewhere in between but are more uniquely used to provide background or contextual information.
Examples:
The people of Seattle, which is known for its rainy weather, were stunned by the forecasted drought.
The people of Seattle — which is known for its rainy weather — were stunned by the forecasted drought.
The people of Seattle (which is known for its rainy weather) were stunned by the forecasted drought.
In this case, the em dashes are the most appropriate, since this appears to be a casual conversation and the aside is modifying the noun Seattle, as opposed to the people or the forecasted drought. Again, context matters. A formal report might be better suited to commas or parentheses. Fiction or narrative nonfiction might be a more fitting choice for em dashes.
Are Spaces Needed?
Commas are connected to the preceding word, with a space following (as in this sentence). Parentheses nestle up to the inserted text, with a space before and after, unless ending punctuation is following the close parenthesis.
Em dashes leave a little more choice up to the writer — there's not a hard and fast rule, unless following a style guide. AP style is used by many media outlets, education streams, and regular writers looking for grammar guidance. When using an em dash, AP style dictates a space on either side. However, outside of AP style, the em dash is traditionally used without spaces. So, for technical, media, or online writing, add spaces. Otherwise, feel free to leave them out.
What About Colons and Parentheses?
Colons can be used in a similar way to a single em dash: to emphasize a final point in your sentence. Parentheses come in pairs (use "parenthesis" for the singular), and can also be used at the end of a sentence, although that’s almost exclusively reserved for pithy, omniscient remarks from a narrator or author.
Examples:
There was only one way this was going to end: in tears.
There was only one way this was going to end — in tears.
There was only one way this was going to end (in tears).
One of these formats will likely jump out as correct for the context. A wry narrator might like the effect of the parentheses. An empathetic journalist might lean toward the colon. A casual email to a friend might riff off the dash.
Semicolons also have a place in the world of stylistic punctuation. The primary use of a semicolon is connecting two sentences that are part of the same thought, but there are some additional cases for the punctuation mark.
The good news? There’s no one rule. Just consider formality and how much emphasis is required, and the right punctuation will flow.