"Whether or not" vs. "whether"

1. • This will depend on whether he's suitable for the job.
   • This will depend on whether he's suitable for the job or not.
   • This will depend on whether or not he's suitable for the job.
2. • It is still not defined whether we're following that approach.
   • It is still not defined whether we're following that approach or not.
   • It is still not defined whether or not we're following that approach.

"Or not" doesn't really seem to be needed to complement "whether". Why do people use it then? Is it redundancy and nothing more? Or is it for emphasis? Or are there cases when "or not" is required for the sentence to be grammatical?

Solution 1:

The addition of the "or not" is neither logically nor grammatically required. I think it's often used conversationally for emphasis. I definitely wouldn't use it in writing myself.

Solution 2:

It's worth pointing out that, etymologically speaking, the roots of whether are which/either of two. It's inherently a "binary choice" word, so whereas "I don't know whether it be fish or fowl" is fine, "I don't know whether it be fish or fowl or good red herring" isn't really grammatical. Which is not to say people never use that extended form - but it does sometimes attract criticism.

With "unary choice" forms such as "I don't know whether I like it", the alternative ("I don't like it") can invariably be shortened to "or not" - or simply discarded completely, since it's implicit anyway.

Possibly some will say if only one choice is presented, you should use "if" rather than "whether", but skimming through written instances of "Tell me whether" suggests that most people have always been quite relaxed on that point.

TL;DR: "or not" is never required if the alternative is a simple negation of the stated proposition, but an "or" clause is required in, say, "You must choose whether to write novels or poems" (presupposing that writing, for example, software is not an option currently on offer).

Solution 3:

The New York Times' stylebook says or not is often redundant.
It is ordinarily omitted when the clause functions as a noun, e.g. it is the object of a verb or preposition, or subject of the sentence.
However, when a whether clause acts as an adverb, or not is needed.
Check this NYT blog post for more details.

Another test, courtesy of Garner’s Modern American Usage: “or not” is necessary when the phrase “whether or not” means “regardless of whether.”

Solution 4:

Similarly to tomothymh, I use "whether" alone unless I intend to convey "regardless of whather." In the latter case, I think "whether or not" is generally preferable to "regardless of whether."

Thus:

Whether I wear a coat depends on the temperature.

But:

I'm going to wear my new red coat whether or not it's cold.