Polyamorous and other multiple-partner relationships are now officially recognized in the city of Somerville, where Mayor Joe Curtatone signed a new domestic partnership ordinance into law on Thursday after a unanimous vote by the City Council.

The ordinance states in part, “The City acknowledges that many laws governing family relationships were enacted in a time when not all families were properly recognized. The City, recognizing its commitment to nondiscrimination and fair treatment of its citizens and employees, adopts this ordinance acknowledging domestic partnerships.”

It then goes on to define domestic partnerships as “an entity formed by people,” not limiting it to a partnership of two.

That language was a change to an earlier draft of the ordinance that Somerville City Councilor Lance Davis worked up in committee.

In fact, what appears to be a first-of-its-kind ordinance in the United States began as a practical domestic partnership policy in response to the COVID-19 crisis.

Davis said Councilors had heard from constituents who were having trouble visiting their loved ones in the hospital.

“They said they were not married, and they said that a domestic partnership ordinance would be something that would help them,” Davis told WGBH News.

Davis said the initial draft was based largely on language from domestic partnership ordinances that nearby Cambridge and Boston had adopted in the years prior to the legalization of same-sex marriage, something Somerville had never done. When Davis sent a draft around for comment from his colleagues prior to last week’s City Council meeting, Councilor J.T. Scott offered an idea.

“He called me, not very long before the meeting, and said, ‘I took a look at this. I just have one question. Why does it only apply to two people?’” Davis said.

Davis talked it through with Scott, and agreed it made sense not to limit domestic partnerships to two people. So did the rest of the City Council.

“When government tries to define what a family is, we haven’t done a very good job of that in the past,” said Davis. “In fact, it’s gone quite poorly at times.”

It is believed to be the first ordiance in the country that officially recognizes multiple-partner relationships in a municipality.

“It’s a true first,” said Diana Adams, executive director of Chosen Family Law Center in New York, which provides legal services to, among others, polyamorous adults. “This is something I have been looking forward to for a decade.”

Adams, herself openly polyamorous, said the stigma of being so is real. She drew comparisons between what’s happened in Somerville to the early years of the fight for same-sex marriage.

“The first civil unions were a really exciting turning point for people in same-sex couples,” she said. “And we’re now at that moment this week with polyamorous couples and other multi-adult families.”

Beyond being a step toward acceptance, it’s tough to know what the Somerville ordinance will mean from a legal standpoint for multiple-partner families, she said.

“This would define domestic partnership and extend rights that are similar to marriage just within Somerville, Massachusetts — which is fairly limited,” she said.

For example, Adams said city employees should now be able to extend health benefits to multiple domestic partners, but it’s unclear whether a private employer in Somerville would have to do the same.

Nevertheless, Adams said she believes this will be the start of a trend, and suspects other municipalities will soon follow Somerville’s lead.

“There are people already reaching out to me from cities like Berkeley [California] and Ithaca [New York] with excitement about how we can work on new potential domestic partnership proposals.”