On Tuesday, San Francisco became the first U.S. city to mandate six weeks of fully paid parental leave. The law is being welcomed as an important step forward for the U.S., which remains one of only two nations that do not have a national paid leave law—the other is Papua New Guinea.
So, what exactly does the new law say—and who will it affect? Here’s a breakdown:
What is it?
The San Francisco law will require employers to provide six weeks of fully paid leave to new moms and dads. It includes same-sex couples as well as adoptive parents.
So it applies to ALL moms and dads in the city?
Well, not quite. According the AP, the law will apply to new parents who work at least eight hours a week and spend at least 40% of their work week in San Francisco. They must also work for a company that employs at least 20 people.
Sign up: Click here to subscribe to the Broadsheet, Fortune’s daily newsletter on the world’s most powerful women.
When does it go into effect?
The first stages of the law kick in in January 2017, when companies that employ 50 or more workers will be expected to comply. Then, businesses with 35 or more employees must comply by July 2017. Finally, companies with 20 or more employees must be compliant by January 2018.
How will it work?
California law already mandates that workers receive 6 weeks of parental leave at 55% of their pay, which is financed by public disability insurance. The new law will require employers to make up the 45% difference.
Who will benefit most?
The law is targeted to help parents in low-wage jobs. While the San Francisco area is known for its big-name tech companies, many of those businesses already offer substantial paid leave policies—Twitter (twtr), for one, announced a new benefit offering 20 weeks fully paid on Tuesday, the same day the San Francisco law was passed.
Who opposes the law?
While the Board of Supervisors passed the measure by a unanimous vote, it did face some opposition. The San Francisco Chamber of Commerce declared itself “neutral” on the law, while the city’s Small Business Commission opposed it, according to NBC.
How does it compare to parental leave laws in the rest of the nation?
San Francisco is the only city in the U.S. to mandate such a policy and California is one of only five states to have some sort of paid leave law on the books. Just last week, New York passed a bill granting 12 weeks of paid family leave for private-sector workers, which will be phased in by 2021. New Jersey and Rhode Island both mandate six weeks of paid leave. Washington passed a similar law in 2007, but has yet to enforce it, according to The Guardian, which also reports that Massachusetts, Connecticut and Washington, D.C. are close to passing paid leave bills.