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I work in the IT industry and often read software and standards specifications that start with a section with definitions for certain words used in the document.

Recently I came across the following in section 1.3 of pdf document SSEK Version 2.0 by Andersson et al [verbatim except for added dashes]:

The keys words SHOULD, MAY, MUST and MUST NOT in this document are to be interpreted as follows:

SHOULD – This word mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
MAY – This word mean that an item is truly optional.
MUST – This word mean that the definition is an absolute requirement of the specification.
MUST NOT – This phrase mean that the definition is an absolute prohibition of the specification.

The question is whether the last definition really is correct, or if it should have been MAY NOT instead to have the defined meaning.

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    Since they are defining the phrases as key words within the context of the document, the definitions are, by definition, correct.
    – JeffSahol
    Jan 26, 2012 at 21:12
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    If the author hadn't gone to the trouble of precisely defining his terms, my logical interpretation of "[some condition] may not [apply]" would be identical to "may" anyway - I'd tend to interpret both versions as meaning "may or may not".
    – FumbleFingers
    Jan 26, 2012 at 21:55
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    The whole point of giving a glossary of definitions is to say that you are using words in a way that may not be quite the conventional meaning. Like a physics book will give technical definitions of "force" and "work"; a computer book may give technical definitions of "memory", "list", etc.
    – Jay
    Jan 26, 2012 at 22:26
  • @JeffSahol said it, Jay said it, FumbleFingers sort of said it too: These are definitions, axioms, terms defined by the author of the document. The author says exactly what each means in the context of the specifications that follow. Seems like a moot point to me. Unless, of course, you are assessing whether or not a revision or update of the document is needed. (Sorry if my tone was harsh, as that might be your motive...)
    – Ellie Kesselman
    Jan 27, 2012 at 1:29
  • The authors are telling you that, in that document, "must not" has that meaning. If they told you "potato" has that meaning, it would still be valid (if a little silly) for them to say so. (The section you quote is common in many specification documents, and likely this one has been copied from another. You're tilting at windmills if you think you can change it.)
    – Hot Licks
    Jan 2, 2016 at 22:03

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The problem here is actually may, not must (or must not). May can mean either optionality or regulation:

I may stop for groceries on the way home tonight.
May I have ice cream for dessert?

Your source is using it in the "optional" sense, not the "regulation" sense, so may not would also mean it's optional. Must not, on the other hand, always means that it is forbidden.

Here are uses that fit with those definitions:

I may stop for groceries on the way home tonight.
I must get gas before work tomorrow or I'll be stranded.
I must not run that red light.

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    One of the major points of using definitions like this is to avoid the phrase may not because it is ambiguous. It can mean either that something might not happen ("I may not go to the movies today if I have too much work to do.") or something that is prohibited ("My mom said I may not go to the movies until I finish my homework.") If they used "MAY NOT" instead of "MUST NOT", they'd fail to accomplish one of their major objectives.
    – David Schwartz
    Jan 26, 2012 at 21:37
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    @DavidSchwartz, most technical writers I know avoid "may not" like the plague for just this reason.
    – Monica Cellio
    Jan 26, 2012 at 21:50
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    Consider it in a technical context: "The network may not change the packet checksum." Is this warning you that the checksum may or may not be changed and thus it might not be the same? Or is this prohibiting the network from changing it and thus guaranteeing that it is the same? This has two possible meanings, one warning that it might not be the same, one guaranteeing that it will be the same. Yuck!
    – David Schwartz
    Jan 26, 2012 at 21:53
  • @DavidSchwartz I understand, and I typically also try to avoid MAY NOT for the same reason. I just wondered, if it would be more English-like compared to MUST NOT... Technical writers have many rather strange/constructed words or phases they use to avoid any misunderstanding of the meaning... But I'm pretty sure my old English teacher would accept this :-)
    – Tonny Madsen
    Jan 26, 2012 at 21:57
  • @TonnyMadsen, but you encountered this in a technical context where precision matters, didn't you?
    – Monica Cellio
    Jan 26, 2012 at 23:16
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I would recommend reading the RFC 2119. They probably tried to abbreviate the definitions:

  1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.
  2. MUST NOT This phrase, or the phrase "SHALL NOT", mean that the definition is an absolute prohibition of the specification.
  3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
  4. SHOULD NOT This phrase, or the phrase "NOT RECOMMENDED" mean that there may exist valid reasons in particular circumstances when the particular behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
  5. MAY This word, or the adjective "OPTIONAL", mean that an item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because the vendor feels that it enhances the product while another vendor may omit the same item. An implementation which does not include a particular option MUST be prepared to interoperate with another implementation which does include the option, though perhaps with reduced functionality. In the same vein an implementation which does include a particular option MUST be prepared to interoperate with another implementation which does not include the option (except, of course, for the feature the option provides.)
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  • I think they may have decided not to allow shall, since that modal verb often leads to confusion in legal documents.
    – Peter Shor
    Aug 17, 2014 at 1:07
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    @PeterShor can you backup your opinion with a reference to some document or discussion? The RFC 2119 alone was not updated nor obsoleted since 1997
    – xmojmr
    Jan 5, 2015 at 8:05
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The use of may not to mean must not makes it sound less authoritative, though it is an abuse of the language. The use of may conveys the meaning of may or may not. So in a military setting, you would expect the use of must not only from the higher authority.

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The definition is correct. The only errors in the quoted section are: “keys words” (should be “key words”) and “this word/phrase mean” (should be “this word/phrase means”).

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  • Ohh yes... Unfortunately that is just your expected errors when Scandinavians write English. We don't have the concept of a 3rd person singular "-s".
    – Tonny Madsen
    Jan 26, 2012 at 21:51
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I would say that it was a good idea for them to disambiguate by choosing "must not" as the opposite to "may"...that is what I think spurred your question, that the opposite of "may" is not "may not", but rather "must not".

Maybe they wanted to keep "may not" as a way to say that the presence of some element is optional...same thing as "may", but with the emphasis on the possibility that it is not present. In that case, outside the "mother may I" sense of the word "may", may/may not are logically subcontraries while may/must not are contradictory.

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