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Comma before “by”: The Definitive Guide

Comma before “by”: The Definitive Guide

Most writers consider commas the quintessential grammatical tool to elevate sentence structuring.

Unfortunately, commas are also one of the most difficult instruments to master, which is why we have put together this comprehensive guide.

In this article, we’ll analyze whether or not it is appropriate to use a comma before ‘by’. Let’s get into it!


Is a comma before ‘by’ ever necessary?

Placing a comma before ‘by’ can be grammatically correct, however, it is rarely encouraged. ‘By’ will often act as a preposition of time, in which case a comma before it is not necessary. Unless there is an inherent need to pause the reader to make the sentence structure more friendly, a comma before ‘by’ should not be included. You should not be separating a sentence with a comma when ‘by’ is followed or preceded by a transitive verb connected to a direct object. A comma should also not be used before ‘by’ within a restrictive clause. Overall, the general rule is that a comma is not necessary before ‘by’. 


The prepositional relationship between ‘by’ and the sentence

As mentioned earlier, ‘by’ is mostly used as a preposition of time or location. You will have used ‘by’ this way on several different occasions.

The below example highlights the correct way to use ‘by’ as a preposition in a professional setting:

Example 1: Mrs. Smith, I will finish the homework by tomorrow.

A comma should not be included when ‘by’ is used as a preposition. The exception to this rule is if you wish for tone to come across in dialogue, as shown below: 

Example 2: Mrs. Smith, I will finish the homework, by tomorrow.


Placing a comma before ‘by’ after a list

You might have also noticed that ‘by’ is used after a list as a preposition. To create a flowing sentence that does not bring confusion to your reader, a comma before ‘by’ should not be colloquially used.

Unnecessarily breaking up a sentence is discouraged in most cases. However, it is not grammatically incorrect to place a comma before ‘by’ in this case.

Example 1: When Marcus came over, he decided to pick up Jess, Annie and Cleo by my house. 
Example 2: When Marcus came over, he decided to pick up Jess, Annie and Cleo, by my house.

Which of these two sentences do you think is easier to understand? 

While neither of these examples are wrong, most people will agree that Example 1 flows more naturally than Example 2. The comma before ‘by’ in this example might emphasize an unnecessary part of the sentence. 


Including a comma before ‘by’ with a transitive verb

For the sake of habitual sentence structuring, placing a comma before ‘by’ when it is concerning a transitive verb is not necessary.

As mentioned above, the tone of the sentence will have to suggest that a comma should be used before one is included.

Commas are a powerful tool used to emphasize tone within dialogue; while colloquial writing may require a comma before ‘by’ with a transitive verb, in academic writing, it is often frowned upon. 

Example 1: We must maintain product quality by ensuring that all machinery is up to standard.  

The example above highlights the appropriate way to include ‘by’ within a sentence in an academic setting,

Example 2: We must maintain product quality, by ensuring that all machinery is up to standard.  

Example 2 is also grammatically correct, however, the tone is more relaxed, so it is not recommended that you use a comma before ‘by’ with a transitive verb in a professional or academic setting.


Should a comma be placed before ‘by’ within a restrictive clause?  

Unlike non-restrictive clauses, restrictive clauses within a sentence always include information that is crucial to the overall context. For this reason, a comma mustn’t be included before ‘by’ within a restrictive clause because it needlessly breaks up the sentence.

Example 1 (CORRECT!): The restaurant by my house is always crowded. 
Example 2 (WRONG!): The restaurant, by my house is always crowded. 

In this case, Example 1 is grammatically correct, while Example 2 is not. Example 2 breaks up the sentence in a way that is not natural in either an academic setting or a casual one.

By placing the comma before ‘by’ in this case, the reader is forced to stop mid-sentence in a way that is abnormal to their instinctual use of tone. 


Tone and sentence structuring

Tone will help you distinguish whether or not a comma before ‘by’ is appropriate. Rhythm is an important component of tone. 

A formal tone is ordinarily used in a professional setting. The key elements of a formal tone are directness, clarity and  contextual logicalness.

Grammatical correctness is emphasized, while the use of pronouns and colloquial phrases is discouraged. On the opposite side of the spectrum, an informal tone can be creative, expressive and relaxed.

Emotions and changes in the voice can be highlighted through sentence structuring that is not always grammatically correct. 



How you decide to place commas will provide you with the power to emphasize information you deem crucial to your reader’s understanding

I hope that you’ve found this guide helpful in explaining the nature of commas before ‘by’, the effect of tone on a sentence, and the relationship between commas and context.