AND/OR (From Wilson Follett, 1966, Modern American Usage, ed. Jacques Barzun)

Whether a lawyer can or cannot make out a case for the necessity of this ungraceful expression in legal documents only a lawyer is competent to say; but anyone else is entitled to the view that it has no right to intrude in ordinary prose. One such intrusion may stand for all: A majority of the tourists come here with camping and/or fishing on their minds. Suppose this to be written with camping or fishing on their minds. How will any sensible reader interpret it? He will presume that some camp without fishing, some fish without camping, and some do both, nothing being said or implied to prevent the three equal possibilities. Note, besides, that these possibilities would be the same if and alone had been used.

We see in this example one of the usual effects of borrowing phraseology from the professions: it kills the plain sense of the words formerly deemed adequate by the layman. That plain sense in the sentence under review is that and can sometimes suggest or, and that generally or includes and. The weatherman’s Snow or sleet tomorrow is no guarantee that we shall have only the one or the other. For generations the chairman has asked Are there corrections or additions to the minutes? well knowing that there may be both. The phrase either… or was invented for situations in which it is important to exclude one of a pair. To be sure, in casual speech you or I must go carries the meaning either you or I; but If he tries that stunt he will be hurt or killed makes it clear that the inclusion of and in or arises naturally from the facts and is habitual for most readers.

Indeed, if the users of and/or were as logical as they pretend to be when they insist on the legalism, they would have to say and or or, since their assumption is that the two cannot co-exist. That assumption is not made better by the punctuation in well, then, there’s Mackenzie and, or, his associates. And if a writer thinks his readers have been so corrupted by the abuse of and/or as to misunderstand his simple or, he should courageously repudiate the hybrid and write – using our first example – tourists come here to camp or fish or both. Let him remember that, except for lawyers, English speakers and writers have managed to express this simple relationship without and/or for over six centuries. This truth is commemorated in the couplet:

Had he foreseen the modern use of and/оr,

It would have sickened Walter Savage Landor.