On the surface, and/or seems like a helpful but mostly harmless little phrase — a little ugly, perhaps, but still useful for those times when you want to be extra clear about what all the options are. Most people associate the phrase with legal writing, but it turns out that a surprising number of lawyers and judges hate it, claiming that it's actually unclear and thus impossible to interpret.

According to Merriam-Webster's Dictionary of English Usage, the construction arose in the 1800s in shipping contracts and became a matter of litigation soon after. Three judges were involved in the first case regarding and/or, and they all came to different conclusions. Later cases ended similarly, mired in confusion over this impenetrable little pair of conjunctions. So what gives? Isn't and/or supposed to increase clarity, not hinder it?

Let's start by looking at the logic of and and or. And is usually used as a logical conjunction, meaning that the statement containing the conjunction is true only if both of the things joined by it are true. For example, if I say I like pie and cake, then the sentence is true only if it's true that I like pie and it's true that I like cake. (I do, so it is.) Or, on the other hand, is a logical disjunction, and this is where it starts to get complicated, because there are two types of disjunctions — inclusive and exclusive.

A statement with an inclusive disjunction is true if one or both of the things joined by it are true, while an exclusive disjunction is true only if only one of the things joined by it is true. If I said You can have pie or cake, the usual implication is that you can have one or the other but not both — that is, it excludes one or the other — but this isn't always so; maybe there's enough that you can have seconds. You can make or explicitly exclusive by pairing it with eitherYou can have either pie or cake.

In other contexts, or tends to be inclusive. If I say If Will or Susan calls, tell them I'm not here, I'd expect you to tell either one of them that I'm not here even if both of them call me. But while we have either–or to make exclusive disjunctions clear, there isn't a good way to make or explicitly inclusive besides wordy constructions like A, B, or A and B or A or B or both. Enter and/or, which seems to be the perfect way to make it clear which kind of or you mean.

Except that, as noted above, some lawyers and judges hate it and say that they don't know how to interpret it. Scott J. Burnham writes, "A number of commercial cases have arisen in which the agreement specified that one party would deliver 'A, B, and/or C' or 'A and/or B and/or C.' The possible combinations are so boggling that it's difficult to tell what was intended." Actually, it's quite easy to list the possible combinations:

  • A
  • B
  • C
  • A and B
  • A and C
  • B and C
  • A, B, and C

As for what the author intended, well, I would assume they intended what they said: one party would deliver one of those options.

Legal writing expert and usage commentator Bryan Garner says that "or includes and" and argues that "the real problem with and/or is that it plays into the hands of a bad-faith reader. Which one is favorable? And or or? The bad-faith reader can pick whatever reading seems favorable." But if or includes and, then "A and/or B and/or C" is exactly equivalent to "A, B, or C." Couldn't a bad-faith reader just as easily pick whatever reading of "A, B, or C" seemed favorable? After all, it seems that the author is presenting them all as acceptable choices. If it's all the same to the author, why not pick the one you like best?

Yet Garner also insists that "if a sign says 'No food or drink allowed,' nobody would argue that it's OK to have both." (Note that this is one case in which or does not include and.) What's to stop a bad-faith reader from doing so here? And strangely, in his Modern American Usage, Garner says that you should use or instead of and/or to avoid ambiguity. But it's or that's ambiguous, allowing either an inclusive or exclusive reading according to context.

I think the real problem with and/or is that it's not always clear if the author really meant to include all those options or if they were just using and/or out of habit. Burnham notes that "a provision limiting liability 'in the event of destruction in the Atlantic and/or Pacific' states an impossibility when read with the conjunctive and," and he suggests rewriting for clarity. In this case it's obvious that the author didn't really mean what they wrote — a ship can't be destroyed in the Atlantic and Pacific.

So while there may be nothing intrinsically wrong with and/or, it may be a sign of sloppy thinking. If you do use it, be sure that you actually mean what you're saying, and also be aware that many editors find it ugly and will replace it with simple or. But more importantly, if you're writing a legal document, keep in mind that many judges hate and/or and may not interpret it the way you intended. Whatever you may gain in clarity is probably lost with the ill will you may engender. You're better off just sticking with or.