For the first time in its 139-year history, the Supreme Court of Canada is being asked to decide whether a judge chosen to join its ranks was lawfully appointed. Prime Minister Stephen Harper chose Justice Marc Nadon in September from the Federal Court of Appeal to fill one of three slots reserved for Quebec on the court. After a legal challenge, Justice Nadon stepped aside without sitting on a single case. Here is a reader's guide to Wednesday's hearing.

The main question: Can a judge be appointed from the Federal Court of Appeal to fill a Quebec slot on the Supreme Court?

What Quebec says: No. Part of the original English-French bargain when the Supreme Court was created in 1875 was that Quebec judges needed to be steeped in the province's Civil Code and culture, it says. That requires a contemporary link to the province, not one that is two decades in the past.

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Who is Marc Nadon? A 64-year-old specialist in maritime law from a village in the Laurentians, whose father was a minor-league hockey player, and who tends to favour judicial restraint.

Why it matters politically: Sovereigntists will make political hay if Quebec loses, says Hugo Cyr, a law professor at the University of Quebec at Montreal. They will cite Maurice Duplessis's dictum, he says, that the Supreme Court is like the Leaning Tower of Pisa – it leans only one way (toward Ottawa). "It's very sad for the institution. It politicizes a process that shouldn't be politicized. As soon as the process of appointment puts in question the legitimacy of one of the judges, it weakens the entire institution. That institution is the institution in our political system that is meant to be above political disputes."

Where the answer lies: Section 6 of the Supreme Court Act sets out who can be appointed to the court's Quebec seats: "At least three of the judges shall be appointed from among the judges of the Court of Appeal or of the Superior Court of the Province of Quebec or from among the advocates of that Province." There is no mention of the Federal Court. Justice Nadon was a member of the Quebec bar for two decades before becoming a judge. But the clause about advocates is in the present tense.

The government's fallback: If the answer is no, Ottawa could simply rewrite the law to expressly permit such appointments – if the Supreme Court says a constitutional amendment wouldn't be needed.

The optics: Justice Nadon swore an oath of office in front of Chief Justice Beverley McLachlin in early October. Now the Chief Justice and the rest of the court will have to make a legal ruling on his appointment. "It looks like a conflict of interest; however, there is no other practical resolution of this problem," says Ian Greene, a professor emeritus of political science from York University.

Harper's gamble: The government knew the law was uncertain, and obtained a legal thumbs-up from retired Supreme Court judge Ian Binnie before making the appointment. But most observers say Mr. Harper underestimated the chances of being challenged. (Toronto lawyer Rocco Galati made the initial challenge.) The result is that the Supreme Court has been sitting with eight members at most, even for an important case on the future of the Senate.

The government's best argument: Common sense. Parliament would not bar someone who had amassed 20 years on the Federal Court, as if that detracted from their experience as a lawyer.

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The best argument against the appointment: A 2002 legal change that removed Federal Court judges from Quebec being eligible to serve as ad hoc judges when the Supreme Court needed an extra Quebec judge to hear a case, according to political scientist Peter Russell of Toronto.