There are many flavors of friendship. Most U.S. adults say they have pals who fit into specific niches in their lives, like gym friends or work friends. These relationships may come and go as life circumstances change, fading away when someone switches jobs or loses interest in a shared hobby.

Then there are close friends, those you lean on in hard times and know on a deeper level. Many U.S. adults say they have only a small handful of friends who fit into this category.

Rarer still are the true forever best friends, those who are by your side for decades on end—through jobs, moves, relationships, fights, losses, and life stages—and may even come to feel like family. But what makes a friendship durable enough to stand the tests of time in this way? 

Shared traits, interests, and backgrounds help a lot, says Robin Dunbar, an evolutionary psychologist and author of Friends: Understanding the Power of Our Most Important Relationships. Dunbar’s work suggests there are seven areas of overlap that are particularly crucial in forming a solid friendship: speaking the same language, growing up in the same area, having similar career trajectories, and sharing hobbies, viewpoints, senses of humor, and tastes in music. Every close friend pair may not have every one of these things in common—but the more they share, the stronger their relationship is likely to be, Dunbar says.

Read More: How to Make Friends as an Adult–at Every Life Stage

Despite the cliché that opposites attract, research actually suggests “we prefer people who are very similar to us,” he says.

Research by Jeffrey Hall, director of the Relationships and Technology Lab at the University of Kansas, also finds that people need to spend lots of time together—at least 300 hours—to become true best friends. And, Hall says, friends who express their deepest thoughts and emotions to each other tend to become more tightly bonded than those who keep it surface level.

Once you’re solidly close with someone, consistency is key to staying that way, says Aminatou Sow, who co-wrote the book Big Friendship: How We Keep Each Other Close with her friend Ann Friedman. Ride-or-die friends don’t necessarily have to see each other all the time, but research does suggest friendship maintenance is important, Sow says.

Assurances about the future—making clear to your friend that you want them in your life for the long haul—and developing shared rituals are good ways of doing that, she says. A “ritual” can be as simple as regularly sending memes or scheduling a monthly phone catchup. Or it can be borrowed from the realms of family and romantic relationships: taking an annual friend vacation, celebrating birthdays and life events together, even marking your friendship anniversary. “These are small things that keep the magic alive,” Sow says. 

And it is indeed “magic,” in Sow’s view. She doesn’t think science has all the answers when it comes to close friendship and why some relationships last forever. “You don’t predict who you fall in love with,” romantically or platonically, she says. “Some of it is mystery and magic and the rest of it is hard work.” 

There is an ineffable quality to some best friendships, Hall agrees. Science suggests it takes a lot of time to build a strong bond—“but what’s very weird,” he says, is that once people become best friends, they may go months or even years without talking and still pick up right where they left off. Sometimes, “once a very strong friendship has been created,” Hall says, “it never really stops being that way.”

How do real-life BFFs explain their decades-long connections? TIME spoke to a lifelong friend pair to find out.

Amy Kohn, 69, who lives in New York, and Madeleine Rudin, 69, who splits her time between Florida and Connecticut, have been friends for 65 years.

MR: We grew up across the street from each other in New York City. We met on the playground and then started kindergarten together the next day. We ended up being in school together for 14 years. We just clicked. 

AK: I never felt like I fit in very well at school, so having a best friend was everything. Madeleine helped me go through the first 18 years of my life. It was always us against the world. 

MR: It became trickier when we went off to college in different states, but we would write letters. I visited once or twice, and we saw each other when we were home on school breaks. 

AK: But then we had a long period where we had no contact. I came out to Madeleine when I was 21 and she was terrific. But I had a number of separate bad experiences socializing in straight environments, and as a result, I became enmeshed in New York City’s gay community from my late 20s into my 30s. We weren’t in touch during that time.

MR: I made other friends. I wasn’t angry at her; I just figured we sort of went our separate ways. And then one day out of the blue, Amy emailed me. 

AK: There’s no good answer to why it took so long for me to do that. I finally did because I had been with my family for Thanksgiving and my cousin asked about Madeleine. The phone rang immediately after I sent the email, and it was Madeleine. Back then, I didn’t know email went that fast! 

MR: The day we re-met for lunch, I remember weeping. It seemed like such a waste of years, because we just clicked immediately again. After that, it never stopped. 

AK: In many ways, we’re very different—I’m all about sports and active stuff and Madeleine isn’t into that. But there’s a level of trust and unconditional acceptance that is the core of everything. I know I can tell Madeleine anything, and if she disagrees, there’s not a scintilla of judgment. Whenever anything really good or really bad happens in my life, immediately, I want to tell Madeleine. She just gets me. If I think I’m being funny, she does too. 

MR: I feel the same way. I would have said the same things about her! I’ve also had a lot of health challenges over the years, and Amy has been there every step of the way. She’s my go-to, other than my son. I know she’s not going to tire of me being ill. She’s just so supportive. 

AK: We’re really explicit about how important we are to each other. We say, “I love you” a lot. We have verbalized that we’ll be there for each other forever and that, at our age, is enormously comforting.

Another piece of advice that I gave my daughter is, “Recognize that all of my friends, on any given day, are idiots.” Meaning, I don’t love everything they do, but I still love them.

MR: I’ve told my son, “You can have friends for different reasons.” Not all of my friends want to do everything that I want to do. For a while, I felt like I had to be as close with everyone as the next person. Then I realized, “No, I don’t.” But with Amy, I don’t have any of those issues.