When used in a nonrestrictive clause, does “which” have to be the first word?

Question

I understand that “that” is used for restrictive clauses, and that “which” is used for nonrestrictive clauses. However, I’m unsure if the placement of “which” in a nonrestrictive clause changes the rules that govern it. Specifically, if “which” is not the first word after a comma, does it alter the rules of the clause in any way? For instance:

Henry was a mechanic, a job which requires a keen eye.

I understand that “… a job…” can be removed and the sentence rewritten. However, is it correct in its present form? Or should “which” be replaced with “that”?

Answer 1

The answer is "No, but it is probably commoner to use "which" in non-restrictive (descriptive) clauses and "that" in restrictive (defining) clauses." (I notice that American English seems to do this more often than British English.)

Non-restrictive (descriptive) clauses should be offset by commas; restrictive (defining) clauses are not. However a job is in apposition to “a mechanic” and is merely descriptive. (This is creates a strange, but grammatical, sentence as everyone knows that being a mechanic is a job.)

Your sentence has several forms:

Henry was a mechanic.

Henry was a mechanic, a job. (A job is in non-restrictive apposition to “mechanic” and the comma is necessary.)

Henry was a mechanic, a job,[note the comma] which requires a keen eye. (A job is still in non-restrictive apposition to “mechanic”* but “which requires a keen eye* now qualifies “mechanic” although it is unclear because of a lack of context as to whether the clause is restrictive or non-restrictive.)

Henry was a mechanic, a job that requires a keen eye. that requires a keen eye is a restrictive clause and “a job that requires a keen eye” is a noun phrase in apposition to "mechanic".