Equal authenticity rule
The equal authenticity rule is a rule of judicial interpretation developed by Canadian courts as a way of interpreting laws which are written in two parallel texts in English and French. The constitution of Canada requires that both versions of each Canadian law be treated as equally authoritative, which can result in problems when the English and French versions do not say exactly the same thing. The equal authenticity rule is derived from section 133 of the Constitution Act, 1867, which states, inter alia, "The Acts of the Parliament of Canada and the Legislature of Quebec shall be printed and published in both those Languages [i.e. both English and French].”
The rule requires courts to treat “both [the English and French] versions of statutes [as] equally valid and authoritative interpretations of the law,” The rule holds that both versions are equally authoritative even if the statute in question was clearly drafted in one language and translated into the other, and even if the two versions of the law are incompatible.
An early version of the principle may be found a statute of the Parliament of the Province of Canada, enacted in 1857. The Act respecting the Codification of the Laws of Lower Canada relative to Civil Matters and Procedure requires that the Civil Code of Lower Canada be printed only in bilingual form, with English and French displayed on the same page: “[T]he two texts, when printed, shall stand side by side.” It has been suggested that this provision of the 1857 Act is "best understood as a non-verbal legislative sign that the two texts should be interpreted dialogically."  Whether or not this is an overstatement, it is certainly true that side-by-side publication was the first step towards the eventual adoption of the Equal Authenticity Rule for interpreting bilingual statutes.
The equal authenticity rule was developed by the Supreme Court of Canada in 1891, in the court’s ruling in CPR v. Robinson. This case dealt with the Civil Code of Quebec, which is enacted in both French and English.
By implication, the rule also related to all laws enacted by the Parliament of Canada, as section 133 of the Constitution Act, 1867 is equally binding upon the federal Parliament and Quebec legislation.
Unresolved issue regarding the application of the rule
Some Canadian provinces (Quebec and Manitoba from the time of their establishment, and since 1982 New Brunswick) are constitutionally obligated to enact all laws in both official languages. However, a number of other provinces, such as Ontario, have opted to enact their laws in both English and French, even though they are not constitutionally required to do so. One question, not yet tested in the courts, "arises...as to whether both versions of bilingual legislation enacted in a province where there is no constitutional requirement to do so would be considered equally authoritative in the absence of an express Equal Authenticity Rule." (An "express" equal authenticity rule would presumably take the form of such a rule in the relevant province's Interpretation Act or some other piece of quasi-constitutional legislation.
Bastarache et al., who raise this question, answer it in the affirmative, explaining that in their view, "the logic applicable to federal, Quebec and Manitoba legislation under the Constitution would also apply [even in the absence of a legislated statement of this logic]: that is, where two languages [sic] versions of a statute are enacted simultaneously both versions will be considered equally authoritative."
- This definition of the equal authenticity rule is from Michel Bastarache, Naiomi Metallic, Regan Morris and Christopher Essert, The Law of Bilingual Interpretation. Markham, Ontario: LexisNexis, 2008, p. 17.
- Michel Bastarache, Naiomi Metallic, Regan Morris and Christopher Essert, The Law of Bilingual Interpretation. Toronto: LexisNexis, 2008, p. vi.
- Michel Bastarache, Naiomi Metallic, Regan Morris and Christopher Essert, The Law of Bilingual Interpretation. Markham, Ontario: LexisNexis, 2008, p. 27.