The Santa Fe New Mexican has a reputation for left-leaning, heavily-biased reporting, but I can’t fault the commas in this article by Tom Sharpe. In his article about alleged pressure to vote for mining rights, I came across this great sentence:
Fielder, who retired last year as a history professor at New Mexico State University, is the only committee member appointed by Richardson who was reappointed by Martinez.
This is a great example of Zen Comma Rule U: Use commas to separate non-restrictive phrases and clauses starting with who.
A non-restrictive phrase or clause renames or restates something just written. It doesn’t provide essential information to understand the main point of the sentence and can be removed without resulting in a non-grammatical sentence. Unlike a restrictive clause starting with who, a non-restrictive clause doesn’t tell us which person we’re reading about. Rather, it provides an additional description.
In the sample sentence, the clause who retired last year as a history professor at New Mexico State University doesn’t tell us which person we’re reading about. It provides additional information. It is a non-restrictive clause.