Recently I posted the following question to the alt.fan.cecil-adams news group on the Internet:
"Yo, grammar mavens! What is the rule governing the use of 'or not' with 'whether'? The following sentences both make sense to me as a native speaker of English: (1) I don't know whether it will rain on Monday. (2) I will see you on Monday, whether or not it rains. Are these sentences grammatically correct?"
RAJ replied: "You're correct; they're both acceptable and proper."
BPH replied: "You're incorrect. The former is not proper, and the latter, while not improper, is verbose, even though it is common. 'Whether' denotes a differentiation between several choices, and should not be used with a single antecedent. The proper word to use for the subjunctive clause in the first sentence is 'if,' as in, 'I don't know if it will rain on Monday.'"
To which FH replied: "On what planet-of-the-hyperactive-alien-schoolmarms, Bub? Thus spake the American Heritage Third: 'whether 1. Used in indirect questions to introduce one alternative: We should find out whether the museum is open.' A usage note under the definition of 'if' specifically discourages the use of 'if' in such cases because it often creates ambiguities."
BPH had concluded his post with the thought, "The worst part about grammar flames is triple-checking to be sure you haven't made your petard self-hoisting." To which FH replied: "Maybe you should've given it one last check before you lit the fuse." Oh, my.
While I'm not exactly sorry I asked, I am not actually any clearer on the concept, and decided I should submit this to the Omniscient One, aka Unca Cece. --Deborah, via the Internet
And smart you were to do so, Deb. I love grammar questions, because they give everybody a chance to get passionate about a matter of no consequence at all, only without the use of guns. They should try this system in the Balkans.
Regarding the question at hand, your sample sentences are acceptable and proper as stated. I would be predisposed to think this regardless: number one, because you show the proper attitude of awe with respect to myself, an all-too-rare occurrence these days, and, number two, because one of your defenders was Frank H., one of Cecil's buds from way back. While Frank is not always right (nobody is, except me), anybody who can come up with a phrase like "planet-of-the-hyperactive-alien-schoolmarms" you gotta love. But notwithstanding my prejudices, you are supported by both authority and common sense.
As you rightly surmise, there are instances in which it is wrong to append "or not" to "whether." The test for determining such instances is whether or not you can delete "or not" without affecting the sense of the sentence. For example, in the preceding sentence "or not" adds nothing to the sense and is thus superfluous, if hard to resist. Not so in your sentence #2. Regarding sentence #1, both Frank and the AH3 are correct in pointing out that though "if" and "whether" are more or less synonymous, "if" can be ambiguous in some circumstances. The AH3 example is "Let her know if she is invited," which can be interpreted to mean "Let her know whether she is invited" or "Let her know in the event that she is invited."
Cecil naturally speaks with popelike infallibility in these matters, but since there are always unbelievers let me quote Theodore Bernstein (The Careful Writer, 1965): "Usually the or not is a space waster. . . . When, however, the intention is to give equal stress to the alternatives, the or not is mandatory. . . . One way [besides Cecil's] to test whether the or not is necessary is to substitute if for whether. If the change to if produces a different meaning . . . the or not must be supplied." Your sentence #2 once again passes the test.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.