Use commas to set off non-restrictive modifiers. Do not use commas to set off restrictive modifiers.
Non-restrictive vs. 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.