Use commas to set off non-restrictive modifiers. Do not use commas to set off restrictive modifiers.
A non-restrictive modifier adds information that is not essential to our understanding of the sentence; if we remove it from the sentence, the basic meaning of the sentence does not change.
A restrictive modifier identifies, or limits the reference of, the noun it modifies.
Example of a non-restrictive modifier:
The statue of his mother by Joseph Smith, dated 1894, sold for over a million dollars.
The date of Joseph Smith's statue can be removed from the sentence without altering its meaning:
The statue of his mother by Joseph Smith sold for over a million dollars.
Example of a restrictive modifier:
The painting dated 1894 is a forgery; the one dated 1892 is genuine.
The phrases "dated 1894" and "dated 1892" cannot be detached from the sentence without making the meaning unclear:
The painting [which one?] is a forgery; the one [which one?] is genuine.
Note the distinction and the different punctuation in the following pairs of sentences:
Non-restrictive William Carlos Williams, the poet, was also a farmer.
Restrictive The poet William Carlos Williams was also a farmer.
Non-restrictive John, who has been drinking, should not drive.
Restrictive People who have been drinking should not drive.
Non-restrictive Many Americans travel to Mexico, where Laetrile is legal and readily available.
Restrictive Many Americans travel to countries where Laetrile is legal and readily available.
Non-restrictive In spring, when the water is high, the lake surges over the rocks.
Restrictive At times when the water is high the lake surges over the rocks.
Non-restrictive The waiters, dressed in their white jackets, are already arranging the chairs on the sidewalk.
Restrictive The waiters dressed in white jackets serve in the main dining room; those in red serve in the coffee shop.
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For further information on commas see our page on Coordinating Conjunctions or our grammar and style FAQ. Or take one of the free grammar, style, and punctuation workshops offered by the Writing Center.