Commas divide a sentence into its components, which are easier to process for the human brain than the complete sentence. A comma is a working aid for the reader. So when a comma is optional, go ahead and use it. It simplifies your text and prevents misinterpretation.
Don’t add a comma just because you “feel” that there should be one. Commas can only guide readers reliably if comma placement follows widely recognized conventions.
The following sections sum up the key rules as briefly as possible. The given examples cover all major scenarios that typically occur in technical writing.
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Insert a comma before the final and and before the final or in a list. The comma is optional here, but it’s helpful.
Yes:
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Before baking a cake, set aside some flour, eggs, sugar and salt.
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Top:
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Before baking a cake, set aside some flour, eggs, sugar, and salt.
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Sometimes a comma changes the meaning of a sentence. If there’s any risk that readers might confuse the facts, use parentheses or restructure the sentence.
No:
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The meeting was organized by John, my boss, and me.
(Unclear: Was the meeting organized by three people—John, my boss, and me? Or was it organized by two people—my boss, whose name is John, and me?)
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Yes:
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The meeting was organized by John (my boss) and me.
or:
The meeting was organized by John, by my boss, and by me.
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Use a comma to separate two adjectives if each adjective modifies the noun alone.
Yes:
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a long, heavy pipe
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Don’t use a comma if the second adjective is essential to the meaning of the phrase.
Yes:
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the local heavy industry
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Use commas to separate main clauses, especially those with but, and, and or. Commas are optional here, but helpful.
Usually there’s no comma when the subject of the second clause is left out because then the second clause is no longer a full-fledged main clause.
Yes:
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The manual is excellent, but a few things are missing.
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Yes:
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Setup analyzes your system, and then it copies the required files to your hard disk.
but: Setup analyzes your system and then copies the required files to your hard disk.
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Don’t be fooled by subordinating conjunctions. Subordinating conjunctions always join a subordinate clause to a main clause. For this reason, if you have a subordinating conjunction, the second part of the sentence is NOT a main clause but always a subordinate clause. So there is NO comma.
The most common subordinating conjunctions are:
▪after ▪although ▪as, as long as, as much as, as soon as ▪as if, as though ▪because ▪before ▪even if, even though ▪how ▪if ▪now that ▪provided (that) ▪since ▪so that ▪that ▪though ▪till, until ▪unless ▪when, whenever ▪where (unless “where” is used in a non-defining relative clause where there must be a comma) ▪wherever ▪while
Yes:
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The indicator light flashes because the door is still open.
(but: Because the door is still open, the indicator light flashes.)
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Yes:
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The measurement starts as soon as you press the Test switch.
(but: As soon as you press the Test switch, the measurement starts.)
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In some rare cases, when a verb is in the negative, you may need to put in a comma or to rephrase a sentence to avoid confusion.
For example, the sentence “The engine did not start because the temperature was too cold.” could mean two things:
▪The engine did not start. The reason was that the temperature was too cold. ▪The engine started, but the reason why it started wasn’t the cold temperature. |
Use commas when you insert an extra comment into the middle of a sentence.
Tip:
When you need to separate ideas with commas, this often indicates that you should split the sentence into two separate sentences or that you should rephrase the sentence.
Yes:
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This command, which can also be used for printing, is only available in the Pro version.
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Top:
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This command is only available in the Pro version. You can also use it for printing.
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Use commas to separate subordinate clauses only if the subordinate clause stands before the main clause.
Yes:
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Choose Save from the File menu to save the file.
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Yes:
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To save the file, choose File > Save.
(Note: This version is better because it states the goal before the action.)
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Commas are optional but helpful after an introductory phrase.
Yes:
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On a sunny day like this, wear sunglasses.
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Yes:
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However, don’t wear dark sunglasses when driving a car.
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Yes:
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After taking off your sunglasses, put them back into the case.
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Yes:
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For details, read chapter ABC.
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Don’t be fooled by a main sentence that looks like an introductory phrase.
Yes:
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When the indicator light flashes, insert new paper.
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Yes:
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Insert new paper when the indicator light flashes.
(“Insert new paper” at the beginning of the sentence isn’t an introductory phrase here, but the main clause. So there’s no comma.)
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When a sentence begins with “Also” or “So,” it’s not always easy to decide whether the words “Also” or “So” are an introductory phrase.
If you can replace “Also” with “In addition,” it’s usually an introductory phrase and thus followed by a comma. If “Also” closely relates to a statement in the previous sentence, it’s not an introductory phrase and there is no comma.
If the word “So” at the beginning of a sentence is essentially just a filler word that could be omitted, it’s an introductory phrase and thus followed by a comma. If the word “So” at the beginning of a sentence suggests some logical continuity, for example, between describing a situation and its usual result, it’s not an introductory phrase and there is no comma. In this case, it would often be possible to combine a “So” sentence with the preceding sentence.
Yes:
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Don’t touch the sensor. Also, make sure that the sensor is positioned vertically above the specimen holder.
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Yes:
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Don’t touch the sensor. Also don’t touch the specimen holder.
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Yes:
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So, now that you know this rule, you’ll make fewer mistakes.
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Yes:
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A car is a lot heavier than a motorcycle. So it consumes more fuel.
or:
A car is a lot heavier than a motorcycle, so it consumes more fuel.
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Use commas before and after for example.
Yes:
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For example, this sentence has a comma.
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Yes:
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This sentence has a comma, for example.
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Yes:
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This sentence, for example, has two commas.
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The following examples are just simple sentences. They’re not a main clause plus another main clause, and they’re not a main clause plus a subordinate clause.
Yes:
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A man of his abilities would always be successful.
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Yes:
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We thought that the machine would run out of fuel.
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Add a comma before a non-defining relative clause. Non-defining relative clauses often begin with which.
Don’t add a comma before a defining relative clause. Defining relative clauses often begin with that.
Important: Make sure that you understand the different uses of that and which. For details, see that / which.
Yes:
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The program now imports the data, which may take several minutes.
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Yes:
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The program now imports the data that you’ve selected.
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Yes:
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This is an optional field, where you can enter some notes.
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Yes:
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This is the place where you must enter your name.
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Note that the presence or absence of a comma can make a great difference in meaning.
Yes:
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Most users of the program, who have little time, don’t read the manual.
(This means that most users don’t read the manual because they have little time.)
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Yes:
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Most users of the program who have little time don’t read the manual.
(This means that usually users do read the manual, but those who have little time don’t.)
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Usually, there’s no comma before that.
Yes:
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It’s important that you read all safety instructions before using the product.
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Yes:
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This is the answer that you’ve been looking for.
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Sometimes a that-clause follows an abstract noun like answer, belief, demand, fact, knowledge, news, proposal, statement, and so on. If the that-clause isn’t necessary to complete the meaning of the sentence, mark the that-clause by commas.
Yes:
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The fact that the device is easy to use makes it suitable for beginners.
(The short version of this sentence is: “The fact makes it easy for beginners.” This doesn’t make sense.)
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Yes:
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The initial presumption, that specimen A was heavier than specimen B, was wrong.
(The short version of this sentence is: “The initial presumption was wrong.” This makes sense. The that-clause is just an addition to this statement.)
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Normally, there’s no comma before if.
However, there’s a comma within a sentence that begins with If.
Yes:
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Your car looks dirty if you don’t wash it.
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Yes:
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If you don’t wash your car, it looks dirty.
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There may be a comma before if when there’s a list of if-clauses, or after an introductory phrase.
Yes:
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If you want to save your settings, if you want to print reports, or if you want to send reports by email, you need to buy the Pro version.
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Yes:
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Therefore, if you still don’t know how to use commas, ….
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There’s always a comma before “too” at the very end of a sentence.
Yes:
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The temperature of the engine is too hot.
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Yes:
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You can learn how to use commas, too.
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