“Under the previous law, people who did receive life sentences in fact served life sentences,” he said.

But even if it brought little in the way of practical changes, Professor Penney said that the law was a significant shift for Canada, where sentences were historically intended to show the public’s disapproval of crimes without slipping into vengeance, or violating the constitution’s ban on “cruel and unusual punishment.”

Judges are still figuring out how it should work, he said.

Friday provided two examples. The judge in Toronto decided that Mr. McArthur will serve his sentences at the same time, citing his age and poor health. If he is still alive, Mr. McArthur will be 91 when he attends a parole hearing.

In Quebec City, prosecutors had suggested that Mr. Bissonnette get consecutive sentences that would have put off his parole hearing for 150 years. The judge compromised and postponed his potential parole for 40 years after concluding that 50 years or more would be cruel and unusual punishment.

That decision only underscored the confusion around the law.

A year ago, Justice Michael Code of the Ontario Superior Court of Justice complained that the law limited him to either extending the parole waiting period to 50 years or keeping it at 25 in a case involving two men he had convicted of a carrying out a second murder.

“These are stark alternatives,” he wrote before encouraging lawmakers to change the law to make it possible for judges to extend the wait for parole as they saw fit.