The most populous state in the nation will soon allow terminally ill patients to end their own lives.

Calling it a "gut-wrenching" decision, California Gov. Jerry Brown (D) on Monday signed into law so-called "right-to-die" legislation, handing the death-with-dignity movement one of its most politically significant victories ever.

Supporters of the law were no doubt helped by one of their highest-profile champions, the late Brittany Maynard, a 29-year-old Californian who in her dying days graced magazine covers and the Internet in late 2014 advocating for right-to-die laws.

Maynard certainly helped swing the tide of public opinion to her side, and it's very possible her victory won't be contained just to California. Here's everything you need to know about California's new medical reality -- and its potential impact on the rest of the nation:

Just four other states have such laws

California joins Oregon, Washington, Vermont and Montana.

That means when California's law goes into effect, likely in early 2016, one in 10 Americans will live in a state that allows terminally ill patients to end their own lives, according to the right-to-die advocacy group Compassion and Choices. 

And it's plausible that more states could soon follow California's lead. At least 20 state legislatures have recently considered similar legislation, and California has been proven as a testing ground for controversial laws in the past.

A big reason this is advancing now: Public opinion would now seem to be on right-to-die supporters' sides, after a clear shift. Americans' opinions on it have changed drastically, Pew Research Center notes. In the graph below, the shift upward  notably happened around the time of or after Maynard's death. Before she became a national figure in late 2014, 51 percent of Americans supported doctor-assisted legal suicide; today about 68 percent do.

As far as shifts on issues like this go, that's a huge and rapid change.

The path to get here wasn't easy

In fact, the law almost didn't come to fruition. A similar proposal had failed several times in the state legislature and on a ballot question. This time around, the right-to-die bill didn't advance in this year's state assembly session, but when lawmakers convened a special health-care session, it got a second chance.

It's opposed by most Republican lawmakers on moral and religious grounds, and even Democrats were split. In fact, according to the Los Angeles Times's Patrick McGreevy, the law divided much of the state: Religious leaders, physicians, ethicists, regular people. Among the dramatic testimony on either side was a dying former Los Angeles police detective, who is unsure if she'll make it to law's effect date but wanted to see it implemented for other people. 

When the bill did pass the state legislature, what to do with it was a tough and incredibly personal decision for California's governor. Brown is a lifelong Catholic and once went to Jesuit seminary school to study to be a priest.  Torn, Brown said he spoke with everyone from his own doctors to Archbishop Desmond Tutu before finally deciding approving the law.

"In the end, I was left to reflect on what I would want in the face of my own death," he wrote in an emotional statement. "I do not know what I would do if I were dying in prolonged and excruciating pain. I am certain, however, that it would be a comfort to be able to consider the options afforded by this bill."

Opponents are worried about abuse of the law

Among those opposed to the law are religious groups and advocates for people with disabilities, who worry it could open the door to all kinds of terrible scenarios, such as premature suicide, coerced early death and overriding God's will. 

Proponents argue there are safeguards against that: Patients must be mentally competent, and their imminent death (within six months) must be confirmed by two doctors.

As for how California's actual doctor-assisted death will work: Doctors prescribe the lethal drugs, patients administer it. A patient must sign off on his or her intention to take the drugs 48 hours in advance and has to physically take the drugs on his or her own.

There's little doubt Maynard played a big role

As we mentioned above, right-to-die laws haven't had much luck in California or anywhere else in recent years, though advocates argue the fact they're even being considered in state legislatures is a plus. It's likely Maynard's case was at least part of the reason the bill finally succeeded.

The young Californian was losing a battle to brain cancer and had decided to end her own life when she and her family came up against the fact that California doesn't allow it. So she and her husband of two years, David, moved to Oregon, where it was legal.

On her way out the door, Maynard filmed a video advocating for right-to-die laws, saying, "I want to make sure no one else has to do what I did and pack up their family and move to another state to not die horribly.”

Like some of the most effective messengers, Maynard was an unusual case for her cause. For one, she was young and her story particularly tragic. The Washington Post's Lindsey Bever points out that in Oregon, where she died, the median age of someone who uses doctor-assisted suicide  is 71. 

In her final days, Maynard traveled to the Grand Canyon, blogged, wrote op-eds on CNN, did photo shoots with People Magazine and partnered with Compassion & Choices. 

Before Maynard died in her bedroom on Nov. 1, 2014, "peacefully, in the arms of her loved ones," she wrote:

"I didn’t launch this campaign because I wanted attention. I did this because I want to see a world where everyone has access to death with dignity, as I have had. My journey is easier because of this choice.”

That choice will now be available to many more Americans.