Prime Minister Justin Trudeau is right to have changed his mind – or at least modified it – on the custom of regional representation on the Supreme Court.

Back in August, Mr. Trudeau said that the court should be more "representative of the diversity of our great country," and was silent on regional representation.

But ever since the Supreme Court was set up in 1875, the practice has been to have three Ontario judges, two Western ones and one from the Maritimes (later the Atlantic provinces), while the rule for three judges from Quebec and its distinct legal system is explicitly carved in statutory stone.

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The Canadian constitution, both small-c and large-C, following the British model, is as much based on unwritten constitutional conventions (though they are abundantly, even incessantly written and talked about) as on the two Constitution Acts, 1867 and 1982.

So the practice of regional representation on the court has become another "unwritten" convention. An opposition MP brought a motion in the House of Commons advocating respect for "the custom of regional representation" on the Supreme Court. It passed unanimously, 270-0; Mr. Trudeau voted for it himself.

What made it all immediately relevant was the retirement of Justice Thomas Cromwell, from Nova Scotia.

It was a motion, however, not a binding law. Afterward, Justice Minister Jody Wilson-Raybould interpreted the motion broadly; she said that the short list for the new Supreme Court judge would include at least one candidate from Atlantic Canada.

Mr. Trudeau's initial point on this is understandable, but the principle of regional representation on the court has put down strong roots, after 141 years; they should not be uprooted now.

The Prime Minister has also rightly emphasized the importance of "functional bilingualism." The upshot will tend, over time, to give a statistical advantage to the appointment of Supreme Court judges who are New Brunswick francophones.

That may not be quite the kind of diversity Mr. Trudeau had in mind, but the perspective of a whole region of Canada is no small matter, much more than one judge. Let's not lose sight of the world view of the four provinces of Atlantic Canada.