We sometimes hear that a U.S. judge has imposed several consecutive prison sentences on a criminal, adding up to more than 100 years.

The Guinness Book of World Records says the longest so-called death-defying sentence handed down for multiple counts in the U.S. was against Charles Scott Robinson, who was sentenced in 1994 to 30,000 years in prison for the rape of six children.

More recently, USA Gymnastics team doctor Larry Nassar was sentenced to between 140 and 360 years for sexually assaulting minors and on child pornography charges.

It is far less common to hear of sentences that long in Canada. But the Canadian system can still ensure some offenders will never get out of prison.

And just like in the U.S., judges here have to decide whether a sentence should be concurrent or consecutive.

What's the difference?

A concurrent sentence means multiple sentences will be served at the same time. In general, this is the rule for multiple convictions stemming from the same event. But the judge does always have discretion.

Consecutive sentences are served one after the other. Sentences for crimes committed on different occasions can be ordered to be served consecutively.

Jaskirat Singh Sidhu, the driver of the truck involved in the crash with the Humboldt Broncos hockey team bus, arrives for the second day of his sentencing hearings Jan. 29. (Ryan Remiorz/Canadian Press)

In Saskatchewan, Jaskirat Singh Sidhu, the semi-trailer driver involved in the Humboldt bus crash, pleaded guilty to 16 counts of dangerous driving causing death and 13 counts of dangerous driving causing bodily harm. He will get 29 sentences, and the judge will decide whether they will be served concurrently, taking into account guidelines in the Criminal Code of Canada and legal precedents.

Remember, Sidhu's offences arose from the same incident, and that is important. The Criminal Code says the sentence for such offences should be served concurrently.

Bruce McArthur will serve a life sentence for eight murders. He'll learn Friday how many years he must wait to request parole. (Bruce McArthur/Facebook)

In the Bruce McArthur murder case — a sentence will be handed down Friday in Toronto — McArthur pleaded guilty to eight counts of first-degree murder. Each count carries a mandatory life sentence. A judge cannot sentence someone to more than one life sentence. So McArthur will receive one life sentence.

What the judge can decide is whether the periods of parole eligibility will also be concurrent or consecutive.

Alexandre Bissonnette will also find out Friday how many years of his life sentence he will have to serve before he can request parole in the shooting deaths of six men at a Quebec mosque. (Facebook/Canadian Press)

The judge deciding Alexandre Bissonnette's sentence for the Quebec mosque shooting faces the same question.

Bissonnette pleaded guilty to six counts of first-degree murder for killing six men, so he, too, will receive the mandatory life sentence. What he'll learn Friday is how many years he'll have to wait until he can apply for parole.

"The judge has an opportunity to add additional periods of [parole ineligibility] time, and depending on how many there are, that would exceed the natural life of the person," says Anthony Moustacalis, former president of the Criminal Lawyers' Association, who's been practising since 1988.

How does adding parole ineligibility work?

As mentioned above, a judge cannot sentence a person to more than one life sentence.

"Life is life," says Toronto-based defence lawyer Sean Robichaud. And just because an offender requests parole at 25 years does not mean the request will be granted.

But there has been a change around parole eligibility in recent years.

This is where it can get confusing.

In 2011, Stephen Harper's Conservative government amended the Criminal Code to allow a judge to "stack" periods of parole ineligibility. For example, Dellen Millard has been convicted of three first-degree murders in Ontario for the separate killings of Tim Bosma, Laura Babcock, and his father, Wayne Millard.

Dellen Millard is serving life in prison for the murder of Tim Bosma. He has been sentenced to three consecutive periods of parole ineligibility, meaning he cannot request parole for 75 years. (Hamilton Police Service/Canadian Press)

He was sentenced to life in prison for the killing of Bosma. He cannot technically serve more than one life sentence. But whereas Millard was eligible to request parole in the Bosma sentence after 25 years, he is now ineligible to request parole for 75 years (three times a parole ineligibility period of 25 years).

People often confuse this and say Millard is serving three life sentences.

"I think that's the point," Robichaud says. "That's the reaction that Parliament wanted for the public to have … the feeling that someone is serving three life sentences. Because in effect, it makes no difference. It's just the change of terminology. Either way he's going to be serving the rest of his life in jail."

McArthur sat silently as friends and family of the eight men he admitted to murdering read victim impact statements in a Toronto courthouse. (Pam Davies/CBC)

The same could happen to McArthur. He will serve one life sentence no matter what. But the judge is scheduled to decide Friday whether to allow him to request parole in 25 years or add more periods of ineligibility.

What about non-murder cases?

When the conviction is not for murder, as in the Sidhu case linked to the Humboldt crash, a judge can decide to sentence consecutively. But as Robichaud points out: "Sentencing is a very complicated and individualized process under the Criminal Code of Canada."

And the Criminal Code states that in general, sentences for offences that arise out of the same event should be served concurrently. The judge could also take into account aggravating factors and use them to add to the sentence.

But here, Moustacalis points out another principle in play: "The judge has to take into account the totality of the sentence, what's called proportionality."

The Criminal Code spells that out in Section 718.2(c), stating that a court should consider that "where consecutive sentences are imposed, the combined sentence should not be unduly long or harsh."

And it spells out in Section 718.3 (4b) the circumstances under which a judge might decide to impose consecutive sentences, including:

The offences did not arise out of the same event,

One of the offences was committed while the accused was on judicial interim release such as parole,

One of the offences was committed while the accused was fleeing from a peace officer.

Again, this does not apply to murder, where one life sentence is mandatory.

What factors could the judge consider?

When sentencing, the Criminal Code instructs a judge to consider "mitigating circumstances relating to the offence or the offender." These can include, but are not limited to, an early plea of guilt.

McArthur's lawyer raised this during the sentencing hearing, arguing the judge should consider, when deciding on parole ineligibility, that McArthur pleaded guilty and avoided putting his victims' families through a long trial.

A judge also considers the impact the offender's crime had on others.

Last week, the judge in the Humboldt case heard victim impact statements. This week, people who knew the eight men murdered by McArthur got their chances to read their victim impact statements. But do they have an impact on the judge's ultimate sentencing decision?

"Someone who's been extremely affected by a crime as a victim, a judge will take that into account," says Moustacalis. "For example, psychological damage to someone or physical injury that's long-lasting is what the court calls aggravating factors, and a victim impact statement can have quite an effect in those situations."

Robichaud agrees a victim impact statement can have an effect, but says, "While it gives the judge a good sense of what happened and the harm it caused, the emotion cannot guide a judge's ruling because that emotion is infinite. And we have to be constrained as to where we set the bar in what an appropriate sentence is."