Commas after introductory clauses: Punctuation tip #1

Did you know that some punctuation rules are optional? Now, I’m not talking about bending the rules in your experimental fiction class or leaving out punctuation in your texts. I’m talking about punctuation that can go either way, depending on your preference.

An obvious choice that comes to mind is the Oxford (serial) comma, particularly in simple horizontal lists. But that's an editorial minefield, so I'll leave it for another post. Today, I'll be covering the use of commas after introductory clauses (i.e. words at the start of a sentence that provide additional information or modify the meaning). But there’s a bit of confusion about when these are optional and when they aren't, so here are 3 quick and dirty rules you can follow:

Rule 1: Prepositions + less than 4 words

If the introductory phrase has a preposition (e.g. in, for, with, between, before, etc.) and less than 4 words (this number depends on your style guide), a comma is optional.

✅ In 1984, I finally bought a boom box.

✅ In 1984 I finally bought a boom box.

✅ On average, I listen to my boom box three times a day.

✅ On average I listen to my boom box three times a day.

This can make your sentence crisp and direct, or provide drama and rhythm. However, when the prepositional phrase has 4 words or more, you’ll need a comma for clarity.

✅ After working for three months, I finally had enough money to buy a boom box.

Rule 2: Ambiguous phrases

A comma is necessary when omission can cause ambiguity or confusion in a sentence. (You don't want to interrupt the flow by making your audience read a line twice!)

❌ In December 2000 boom boxes were stolen from a warehouse.

✅ In December, 2000 boom boxes were stolen from a warehouse.

Without a comma, you’re left wondering if 2000 is a date or the number of stolen boom boxes. If you're referring to 2000 boom boxes, then you need a comma after 'December'.

Rule 3: Adverbs, names and greetings

If the introductory phrase has an adverb (e.g. however, ideally, unfortunately, meanwhile, furthermore), a name or a greeting, you’ll also need a comma.

✅ However, I forgot to plug in the boom box.

✅ Roberto, don’t forget to plug in the boom box.

✅ Goodbye, Roberto.

However, 'Hi Roberto' is acceptable in informal writing, such as emails, text messages and social media. In those contexts, the comma looks fussy and stilted. However, if you're writing dialogue in a novel (even if the language is informal), a comma can add clarity to the sentence.

Commas after introductory clauses: Summing up

Introductory clauses can be tricky, and most writers don't know when they should or shouldn't include a comma. However, if you follow the three simple rules above, you'll make your writing clearer and avoid distracting ambiguities.

When a comma is optional, I try to retain the author’s preference (unless it causes confusion). That’s just good manners! In my own writing, I lean toward using commas in all introductory phrases, even when they’re relatively short. It keeps things simple.

How do you navigate commas in introductory phrases? Do you prefer minimal punctuation? Or maybe you want to talk about my obsession with retro boom boxes. Let me know in the comments below.

For more punctuation tips, see What's a comma splice (and when can you use it)? and How to hyphenate compound adjectives.