In the late ’90s, Antje Danielson’s son Max and Robin Chase’s daughter Linnea often played together on a tire swing in Cambridge, Massachusetts' Dana Park. The two women had met at their kids’ kindergarten, but supervising the playground is how they got to know each other. As Danielson, a Harvard geochemist, and Chase, an MIT business school graduate turned stay-at-home mom, kept chatting, their casual park encounters grew more profound. Chase started telling Danielson about wanting to put her business degree to good use, about her entrepreneurial ambitions. Danielson spoke of wanting to branch out of academia, a desire that had her, too, mulling entrepreneurship. “I was sitting in the playground after school, and there was this other mom who had a business degree and I was telling her about [wanting to start a business],” Danielson recalls. “She said, ‘Oh, that’s really interesting, and I’ve been thinking of starting a company, too.’” Danielson’s husband was encouraging. “He said, ‘Well, why don’t you just ask her if she wants to start this company with you?’”

So, one afternoon in October of 1999, Danielson took a chance and told Chase about her car-sharing idea. Chase was enthusiastic, but she wanted to make sure her husband would be okay with her taking on such a big project. That night she went home, talked to him, and decided to go ahead. Within a few days Chase and Danielson had their first official Zipcar business meeting.

Today, Zipcar — which is still headquartered in Boston — has offices in more than 26 American cities and 860,000 members across the US, Austria, Canada, Spain, and the UK. And the company’s profile only grew when car-rental giant Avis bought Zipcar for $491 million in January 2013. But in fact, both founders left the company more than 10 years ago, as power struggles and disputes prevented both Chase and Danielson from seeing their shared vision through. Now 56, Danielson hasn’t spoken to Chase in more than a decade.