Common Comma Mistakes in Business Writing

Commas are small, but getting them wrong in a work email changes how confident and polished your writing looks, even when the actual content is correct. Cambridge Dictionary's grammar reference lays out the core comma rules clearly, and a few of them cover the exact mistakes that show up most often in business writing.

Commas separate items in a list

This is the rule most people already know, but it is worth restating because the small details still trip people up. When you list three or more items in a sentence, each item gets separated by a comma, with "and" or "or" typically joining the final two items.

Example: "The report covers sales, marketing, and operations." Notice the comma before "and operations." This is called the serial (or Oxford) comma, and while some style guides treat it as optional, using it consistently in business writing avoids genuine ambiguity in sentences where leaving it out could create a misreading, for example "I'd like to thank my colleagues, the client and the vendor" can be misread as meaning the colleagues are the client and the vendor, while adding the comma ("my colleagues, the client, and the vendor") makes the three groups clearly separate.

Commas after introductory words and phrases

Cambridge's grammar reference explains that a comma generally follows an introductory word or phrase before the main part of the sentence begins. This includes transition words like "However," "Therefore," and "In addition," as well as longer introductory phrases.

Compare: "However the budget was approved." versus "However, the budget was approved." The version without the comma can briefly read as "however the budget was approved" (meaning, in whatever way the budget was approved), before the reader realizes "However" was meant as a standalone transition word. The comma removes that brief confusion entirely.

The same applies to longer introductory phrases: "After reviewing the proposal, we have decided to move forward." The comma marks the end of the introductory phrase and the start of the sentence's main clause.

Commas with "which" versus no comma with "that"

This is one of the more technical rules, but it changes meaning, not just style. A clause introduced by "which" and set off with commas adds extra, non-essential information. A clause introduced by "that," without commas, provides essential information that identifies exactly what is being discussed.

Compare: "The proposal, which we sent last week, was approved." (You are giving extra background information; there was only one proposal, and the "which" clause is a side note.) Versus: "The proposal that we sent last week was approved." (You are specifically identifying which proposal, implying there may have been others; the "that" clause is essential to knowing which one.)

In practice, if you can delete the clause and the sentence's core meaning stays intact, use "which" with commas. If deleting the clause would make the sentence unclear about what you are referring to, use "that" without commas.

Commas joining two complete sentences

When you join two complete sentences (each one capable of standing alone) with a conjunction like "and," "but," or "so," a comma goes before the conjunction, not after it.

Correct: "The deadline moved up, and the team adjusted the schedule." Here, "The deadline moved up" and "the team adjusted the schedule" are each complete sentences on their own, so the comma plus conjunction correctly joins them.

A frequent mistake is placing a comma where there is no second complete sentence: "The deadline moved up, and immediately." The second half ("and immediately") cannot stand alone as a sentence, so no comma is needed there at all; the sentence should simply read "The deadline moved up immediately."

Why this matters more in business writing than casual writing

PlainLanguage.gov's official guidance for writing clearly, maintained through Digital.gov, emphasizes that plain, correctly punctuated writing reduces the number of times a reader has to reread a sentence to understand it. In a work context, where readers are often skimming quickly between other tasks, a misplaced or missing comma is one of the most common reasons a sentence needs a second read, which slows down exactly the kind of fast, clear communication that business writing is supposed to achieve.

A quick self-check before you send

• Lists of three or more items: comma after each item, including before the final "and" or "or."
• Introductory words and phrases ("However," "After reviewing X," "In addition,"): comma right after them, before the main sentence begins.
• "Which" clauses: use commas around them if the information is extra background, not essential to identifying what you mean.
• "That" clauses: no commas, and use "that" (not "which") when the clause is essential to specifying exactly what you are talking about.
• Joining two complete sentences: comma goes before the conjunction ("and," "but," "so"), only if both halves could stand alone as full sentences.

Key takeaways

• Use a comma before "and" or "or" in a list of three or more items to avoid ambiguity, even though some style guides treat this serial comma as optional.
• Place a comma directly after introductory transition words and phrases, before the main part of the sentence begins.
• Reserve "which" with commas for extra, non-essential information, and "that" without commas for information that identifies exactly what you mean.
• When joining two complete sentences with "and," "but," or "so," the comma goes before the conjunction, not after.
• Clear punctuation reduces how often a reader has to reread your sentence, which matters especially in fast-moving workplace communication.