And/or

Au regard de la jurisprudence des pays de common law, peu d'expressions ont semble-t-il occasionné autant de litiges que l'expression and/or.

Voir notamment la décision rendue par la Superior Court of New Jersey dans l'affaire In Re: the Estate of Massey (1998) :

"The simply stated, yet deeply, perplexing issue raised involves a bequest in a Will which directed that one-third of the residue should pass “to my niece, DIANE HALL and/or Grandniece, CARLY HALL.”   Beyond the shadow of any doubt, the use of the phrase “and/or” in that setting is utterly ambiguous and convolutes the ascertainment of the decedent's intent.

(...)

What, after all, does “and/or” mean?  “And” is easily defined, as is “or”.   There is, however, no known understanding as to what “/” means.   Does “/” mean “and” or does it mean “or”?   In other words, does “and/or” mean “ ‘and’ and ‘or’ ” or does it mean “ ‘and’ or ‘or’ ”? It is conceivable that if, in the present context, the court should translate “and/or” as “ ‘and’ or ‘or’ ”, see, The Random House Dictionary of the English Language (unabridged, 1967) at 55, then some sense could be divined from this unfortunate will.   Using that definition of “and/or” as a Rosetta stone for deciphering the meaning of the devise in question, the result is:  “I give all my property to A and B, or A or B”.   That, too, sounds peculiar but it could conceivably be translated further as “I give to A and B, but if one of them should predecease me, then I give to the survivor.”

(...)

In this case, with admitted great unease, there would appear to be no doubt that the testator intended “and/or” to be given the meaning, suggested in the grammatical analysis set forth above, that Diane Hall and Carly Hall would equally share in one-third of the residue if they both survived the testator (and, if one of them did not survive the testator, then the other would receive the entire bequest)."

Texte complet de la décision : http://caselaw.findlaw.com/nj-superior-court/1385944.html.

Certains auteurs n'hésitent toutefois pas à prendre la défense de cette expression ou, tout du moins, à lui trouver une certaine justification dans certains contextes : 

« Judges and legal-writing commentators have fulminated against use of and/or, but it has gained greater acceptance among general authorities. It does, after all, have a specific meaning—X and/or Y means X or Y or both. (...). On the other hand, X or Y or both is generally clearer than and/or. And drafters sometimes use and/or when the only possible meaning is that conveyed by or: Acme shall incorporate Subsidiary in Delaware and/or New York. On balance, it is best to avoid and/or. That said, in some contexts and/or is the most efficient way to incorporate into a provision the concept of or both. » (Revisiting the Ambiguity of ‘And’ and ‘Or’ in Legal Drafting, K. Adams & A. Kaye, St John’s Law Review, 2006).