Essential vs. Non-Essential Statements

31. When Do I Use Commas?

E. In parenthetical or non-essential elements of a sentence

I. Non-essential components of a sentence are pieces of information that add clarifying or qualifying information to the main subject of the sentence, but that do not change the basic meaning of the sentence or thought. These are statements that if removed entirely would not adversely affect the readability or meaning of the sentence as a whole.

• Example: “The ancient shaduf , a tool designed to raise water between sources of differing elevations, allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”

The sentence would remain the same without the non-essential phrase between commas: “The ancient shaduf allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”

II. Be careful about identifying between essential clauses (like those beginning with ‘that') and non-essential clauses (typically those beginning with ‘which'). The author must choose whether information is essential to the sentence or not, and then so indicate to the reader by employing the proper conjunction and punctuation. When in doubt, purposely use ‘that' for essential information and ‘which' for non-essential information.

• Example: “The ancient shaduf that raised water between sources of differing elevations allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”
• Example: “The ancient shaduf , which raised water between sources of differing elevations, allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”

Notice that 'which' introduces a non-essential statement, while 'that' introduces an essential one. Since the statements are the same in these examples, the conjunction you choose and the comma placement tells the reader whether or not the statement you are making is essential to the overall thought.

“That” and “which” are also employed, along with “who,” in relative clauses. These can be treated the same as essential and non-essential (sometimes called restrictive and non-restrictive relative clauses), but these will be dealt with at greater length in section 34.

III. Similarly, statements that serve an important purpose but do not necessarily modify or clarify the subject at hand ought to be enclosed in commas. These statements are called parenthetical thoughts and are to be treated the same as non-essential clauses—since both components of a sentence can be removed without ruining the meaning of the main sentence, they need commas to clarify their comparative irrelevance to the main thought.

• Example: “His only thought, if he had one at all, was for his own well-being.”
• Example: “The USU Aggies, like most all fans of collegiate sports, are voraciously loyal to their team.”

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I.	Non-essential components of a sentence are pieces of information that add clarifying or qualifying information to the main subject of the sentence, but that do not change the basic meaning of the sentence or thought. These are statements that if removed entirely would not adversely affect the readability or meaning of the sentence as a whole.
	• Example: “The ancient shaduf , a tool designed to raise water between sources of differing elevations, allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”
	The sentence would remain the same without the non-essential phrase between commas: “The ancient shaduf allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”

II.	Be careful about identifying between essential clauses (like those beginning with ‘that') and non-essential clauses (typically those beginning with ‘which'). The author must choose whether information is essential to the sentence or not, and then so indicate to the reader by employing the proper conjunction and punctuation. When in doubt, purposely use ‘that' for essential information and ‘which' for non-essential information.
	• Example: “The ancient shaduf that raised water between sources of differing elevations allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.” • Example: “The ancient shaduf , which raised water between sources of differing elevations, allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”
	Notice that 'which' introduces a non-essential statement, while 'that' introduces an essential one. Since the statements are the same in these examples, the conjunction you choose and the comma placement tells the reader whether or not the statement you are making is essential to the overall thought.
	“That” and “which” are also employed, along with “who,” in relative clauses. These can be treated the same as essential and non-essential (sometimes called restrictive and non-restrictive relative clauses), but these will be dealt with at greater length in section 34.

III.	Similarly, statements that serve an important purpose but do not necessarily modify or clarify the subject at hand ought to be enclosed in commas. These statements are called parenthetical thoughts and are to be treated the same as non-essential clauses—since both components of a sentence can be removed without ruining the meaning of the main sentence, they need commas to clarify their comparative irrelevance to the main thought.
	• Example: “His only thought, if he had one at all, was for his own well-being.” • Example: “The USU Aggies, like most all fans of collegiate sports, are voraciously loyal to their team.”