I know that in formal contexts, the construction and/or is very ugly and undesirable (and there are many questions here that deal with said formal usage).

Has it become acceptable to use it informally, however, as a sort of rhetorical faux formalism or something else altogether?

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    What does "acceptable" mean here? Obviously people do use the form, both in speech and writing, so I think this is Not Constructive. Jan 17, 2013 at 15:51
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    It's just poor style. Use it wherever you want, however, unless the style manual you must follow forbids it. Technical writers & other writers who don't care about writing ambiguously use it all the time. Biomedical articles are filled with such garbage lazyisms, as are the works of all hack academic writers. Informally, anything goes. Formally, the assigned style manual rules. Ultimately, however, the buck stops at the writer's fingers (OK, "input device"). Writers who respect words & language don't abuse them. The only time Medusa isn't ugly & petrifying is when she's invisible.
    – user21497
    Jan 17, 2013 at 15:52
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    @BillFranke: The only time Medusa isn't ugly is when she's invisible is such a great comment, I wish my upvote helped your reputation. As it is, the best compliment I can pay you is to steal the phrase. Jan 17, 2013 at 16:06
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    @Tim: Thank you. Be my guest. My reputation here's a mixed bag. Sometimes I'm up & sometimes I'm down. I often sabotage it as I build it. No matter. Reality's a mixed bag too. But it is nice to know that some folks like some of what I say. :-)
    – user21497
    Jan 17, 2013 at 16:23
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    @TimLymington but if others then steal it from you, then we'll have to agree to remember where you got it so we can answer the question "What does the idiom 'medusa isn't ugly when she's invisible' come from?" in a few years' time. I disagree with Bill on one point - writers who respect language might abuse them, but they will at least know when it's an abuse.
    – Jon Hanna
    Jan 17, 2013 at 17:00

3 Answers 3


The Chicago Manual of Style [section 5.220] has this to say:

and/or. Avoid this Janus-faced term. It can often be replaced by and or or with no loss in meaning. Where it seems needed [take a sleeping pill and/or a warm drink], try or . . . or both [take a sleeping pill or a warm drink or both]. But think of other possibilities [take a sleeping pill with a warm drink].

There are always more graceful ways to phrase a sentence without resorting to and/or.

  • What I mean is, if I'm specifically aiming for that sort of trip-on-your-laces feel, could I use it?
    – Joe Z.
    Jan 17, 2013 at 16:41
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    @Joe Zeng: If you're happy to use terms like trip-on-your-laces feel, I think you can probably use and/or. At least the latter has the advantage of being generally understood, which I can't believe applies to the former. Jan 17, 2013 at 16:58
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    Strangely, I can hear it being used in a sentence of breezy informality, or to lend a sense of faux pomposity (to wit, voiced by the sort of character who would use the phrase "to wit").
    – Gnawme
    Jan 17, 2013 at 16:59
  • Joe, you mean you want to create an impression of pomposity for a windbag character or something? Heavy use could definitely help that, especially if the cases are such that the context makes it particularly clear - or better yet, makes the either the and or the or part of it an impossibility ("planning to go into space and/or to the moon").
    – Jon Hanna
    Jan 17, 2013 at 17:04
  • Pomposity might not be the right word. I think Gnawme's "breezy informality" says it better.
    – Joe Z.
    Jan 17, 2013 at 17:54

It's appropriate in a technical or legal context (which would often be a type of formal context), where the precision of indicating that you mean "X or Y or (X and Y)" is desirable.

"X or Y" can be validly interpreted as "X or Y or (X and Y)" or as "X or Y but not (X and Y)". Mostly we get by judging which is meant from context, but in legal and technical contexts such misinterpretation could be costly, if not disastrous.

Such contexts also tend to be explicit in the other direction - if they mean "X or Y but not both" they will make sure to include that "...but not both". They may also emphasis an either and an or with bolding or similar.

Such contexts are "formal" by some standards, but different to some other types of formal prose, in which it should probably not be used. This can include different passages in the same work: It may be appropriate in an item list giving a break-down of a process, but not appropriate in accompanying paragraphs.

In informal use, you can of course do whatever you want. Such use would be rather slangy, and like all slang some people will like it and some will not.

  • Do you have a source to support that X or Y does not include X and Y? You describe what is generally known as exclusive or. The inclusive version, formally known as inclusive disjunction, informally known simply as or does include X and Y.
    – JJJ
    Jun 28, 2018 at 16:29
  • @JJJ No, because I didn't say that.
    – Jon Hanna
    Jun 28, 2018 at 16:36
  • You wrote (emphasis is mine): "X or Y" can be validly interpreted as "X or Y or (X and Y)" or as "X or Y but not (X and Y)". To me that seems you say it's valid to interpret or as the exclusive or.
    – JJJ
    Jun 28, 2018 at 16:41
  • @JJJ Ah, I thought you were saying that I was saying that that was the only valid interpretation. Calling exclusive or, exclusive or also implies that it's valid to interpret or as exclusive or, since it uses or as the main term and the qualifier exclusive to clarify which among a choice of possibilities is intended
    – Jon Hanna
    Jun 29, 2018 at 11:24

Look at this entry in Wikipedia:

And/or (also and or) is a grammatical conjunction used to indicate that one or more of the cases it connects may occur. For example, the sentence "He will eat cake, pie, and/or brownies" indicates that although the person may eat any of the three listed desserts, the choices are not exclusive; the person may eat one, two, or all three of the choices.

As you said you can easily use that informally, but you can also use it in formal conditions, where it makes sense. But I think maybe "either" is the best choice for such situations.

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