When they were 11 years old, Kylee and Starr split a stolen Coors and made a wordless pact, pricking their fingers with a cactus needle to let their blood run together. The gesture symbolized the girls’ hope — their determination — that their lives would always be joined just as closely.

“You had this idea that I’ll never forget,” Kylee told Starr much later on. “People move places and change careers for their spouses. And you said to me, ‘Why can’t we do that for our friendship?’ ”“We didn’t get reinforcement for that idea,” Starr said. “We got a lot of pats on the head and ‘Oh, that’s a nice idea, girls, but life happens.’ ”It does. And often the only people who go the turbulent distance with you, there at almost every critical juncture, are the ones who wear tags like mother or father, sister or brother, husband or wife. They have more motivation and more of an obligation to stick.

But sometimes it works out differently. Kylee and Starr each went on to marry and have kids, but more than 25 years after their pact, when they sat down in 2007 to speak about it, they were living on the same street, sharing the burdens that needed sharing and no more able to envision separate existences than they had been when they drank that illicit beer. If that’s not family — real family — please tell me what is.

I read about them in a new book, “Ties That Bind,” which showcases conversations like theirs from the StoryCorps project, an evolving oral history that records pairs of individuals talking about the sacrifices each has made for the other, the favors bestowed, the forgiveness granted. Dave Isay, the founder of StoryCorps, culled about 40 of these encounters for the book, which coincides with the project’s 10th anniversary.

A slight majority of the pairs in the book are linked by DNA, marriage or such. They’re kin in the conventional sense. But what struck me most forcefully was how many others had found extraordinary, enduring intimacy outside of that context, stretching the definition of family, making clear that it’s not just or even chiefly about common genes, common beds.

It’s about common needs, common generosity. It’s an act of will as much as an accident of birth. That’s worth remembering during this merrymaking, reunion-heavy season, when “family” is usually invoked in terms too narrowly traditional. They fail to recognize that former schoolmates, fellow churchgoers, neighbors or other friends can mean every bit as much to you as any actual relatives do. They fail to acknowledge how many people have been let down by those relatives, and have forged a family of their own invention.

That’s true of Chelda, who was 24 in 2009, when she and her best friend, Georgia, two years older, recorded the conversation that appears in “Ties That Bind.” “I remember growing up and picturing this fairy-tale life,” Chelda said, mentioning two of television’s happiest tribes, the Brady bunch and the Huxtables. “ But I didn’t have the family network that I wanted.” She decided that Georgia would fill the gap. She clung to her. And when she got pregnant unexpectedly during graduate school, it was Georgia who rushed to her side and stayed there.

As good as we humans are at division, we’re better still at connection. “Ties That Bind” shows this again and again, even presenting the astonishing story of a woman in constant contact with the man who killed her only son and served 17 years behind bars for his crime. At the time of their recorded conversation in 2011, he was out of prison and living next door to her. She was calling him “son.” And he was professing his love for her and helping to fill the very hole in her life that he, with a bullet, had created.

“Our relationship is beyond belief,” she said.

It certainly didn’t follow any predictable script, and neither did the relationship that Tim, a former nurse, has with Barbara, whose husband, a quadriplegic he once tended to, bringing the couple a crucial comfort. When Tim later developed AIDS, Barbara assumed the role of caregiver and moved him into her home. She spoon-fed him six times a day. They, too, are like mother and child.

In the book there are teachers and students whose closeness transcended and outlasted the classroom. There’s a mentally challenged woman, Janice, who has lived for more than a decade with Sadie, the friend who essentially rescued her from a family that used her as an unpaid housekeeper, seldom let her out into the world and went so far as to have her sterilized. The tenderness between the two women is palpable and breathtaking.

“Your house is the White House to me,” Janice told Sadie last year, when their conversation was recorded.

“We’re not biologically tied,” Sadie said. “We are spiritually tied.” Lucky for them, lucky for all of us, that twine can be as thick as blood.