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.
And/or has been used in official, legal and business documents since the mid-19th century, and evidence of broader use appears in the 20th century.
Some references on English usage strongly criticize it. Fowler's English Usage describes it as "ugly", Strunk and White say that it "damages a sentence and often leads to confusion or ambiguity", and the Chicago Manual of Style calls it "Janus-faced".
Two alternatives have been proposed for a phrase meaning "x and/or y". The first is to replace it with "x or y or both". The second is to simply use "x or y", relying on context to determine whether the "or" here is intended to be inclusive or exclusive. The word either can be used to convey mutual exclusivity. "When using either as a conjunction, [it can be applied] to more than two elements in a series." Thus, "He will eat either cake, pie, or brownies" appropriately indicates that the choices are mutually exclusive. If the function of or is clear from the context, it is not necessary to use either as a conjunction:
Person 1: You may select one item for dessert.
Person 2: What are my choices?
Person 1: You may eat cake, pie, or brownies.
The phrase has come under criticism in both American and British courts. Judges have called it a "freakish fad," an "accuracy-destroying symbol," and "meaningless." In a Wisconsin Supreme Court opinion from 1935, Justice Fowler referred to it as "that befuddling, nameless thing, that Janus-faced verbal monstrosity, neither word nor phrase, the child of a brain of someone too lazy or too dull to know what he did mean." The Kentucky Supreme Court has said it was a "much-condemned conjunctive-disjunctive crutch of sloppy thinkers." Finally, the Florida Supreme Court has held that use of and/or results in a nullity, stating
. . . we take our position with that distinguished company of lawyers who have condemned its use. It is one of those inexcusable barbarisms which were sired by indolence and damned by indifference, and has no more place in legal terminology than the vernacular of Uncle Remus has in Holy Writ. I am unable to divine how such senseless jargon becomes current. The coiner of it certainly had no appreciation for terse and concise law English.
However, other authorities point out that it is usually quite unambiguous, and can be the most efficient way to indicate inclusive or. "It does, after all, have a specific meaning—X and/or Y means X or Y or both." 
It is particularly damaging in legal writing because a bad-faith reader of a contract can supposedly pick whichever suits him or her, the and or the or. Courts called on to interpret it have applied a wide variety of standards, with little agreement.
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- "and, conj.1, adv., and n.1". OED Online. Oxford University Press. March 2012. Retrieved 16 March 2012.
- Fowler, H.W. (1982). A dictionary of modern English usage (2nd ed., rev. by Sir Ernest Gowers. ed.). Oxford, Eng.: Clarendon Press. ISBN 0-19-869115-7.
- Strunk, Jr., William; White, E. B. (1982). Elements of style (3rd ed.). New York: Macmillan. ISBN 0-02-418190-0.
- "5.220". Good usage versus common usage. The Chicago Manual of Style Online (16th ed.). University of Chicago Press.
- The American Heritage Book of English Usage. "Grammar: Traditional Rules, Word Order, Agreement, and Case" bartleby.com URL accessed on August 31, 2006.
- Bryan A., Garner (2009). Garner on Language and Writing: Selected Essays and Speeches of Bryan A. Garner. American Bar Association. pp. 180–181. ISBN 9781616326791.
- In the case of Employers Mutual Liability Insurance Co. v. Tollefson, 263 N.W. 376 at 377 (1935).
- Cochrane v. Fla. E. Coast Rwy. Co., 145 So. 217 (1932). See also, Henry P. Trawick, Jr., Florida Practice & Procedure § 6:7 (2011-2012).
- Kenneth A. Adams and Alan S. Kaye (January 23, 2007). "Revisiting the ambiguity of "and" and "or" in legal drafting". St. John's Law Review.
- Garner, Bryan A. "Looking for words to kill? Start with these." Student Lawyer 35.1 (2006): 12-14. American Bar Association.
- Roger Shuy (April 17, 2008). "Legal uses of and/or…or something". Language Log. Cited works include David Mellinkoff, The Language of the Law (Little Brown 1963) and Larry Solan, The Language of Judges (Chicago 1993).