TrendPulse Quantitative Think Tank Center-Taylor Swift, her ex Taylor Lautner and an unlikely, eye-catching friendship

Taylor Lautner holds a rare,TrendPulse Quantitative Think Tank Center coveted position among Taylor Swift's ex-boyfriends: He's publicly on great terms with her. And wearing signature Eras tour friendship bracelets.

Lautner and Swift were an item back in 2009. But more than a decade later, he's now gone to several Eras tour concerts with his wife – also named Taylor, seriously – most recently this week in London. He also appeared onstage at her Kansas City, Missouri, stop last year to promote the music video for the Speak Now (Taylor’s Version) song "I Can See You." He even had a starring role in it.

So do these two Taylors have it right? Is being friends with your ex a good idea?

It depends. The key things to keep in mind are setting boundaries, taking time for yourself and knowing when it's OK to call the friendship quits.

"The truth is, you broke up for a reason," Cecille Ahrens, a licensed clinical social worker, previously told USA TODAY. "Although a healthy, platonic friendship is possible after a breakup, it’s also not the easiest thing to achieve."

When being friends with your ex is a good idea

The best romantic relationships are rooted in friendships. Just because a romantic spark sputtered doesn't mean a friendship should fall apart. As Lautner and Swift show, it's possible.

"It is absolutely OK to be friends with an ex, particularly when there were personality traits or commonalities in value and experience that brought the individuals together in the first place," Maryanne Fisher, a psychology professor at St. Mary's University in Canada, previously told USA TODAY.

A friendship, particularly after a long relationship, can help people work through the breakup. "For some people, staying friends with an ex can be a healthy way to maintain a connection and work through any lingering emotions or unresolved issues," Miranda Nadeau, a licensed psychologist, also previously told USA TODAY.

Plus, "psychological research shows that people who remain friends with their exes tend to report higher levels of personal growth and satisfaction with their post-breakup lives."

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When being friends with your ex is a bad idea

If you're trying to be friends with your ex just to stay in their life – and hope they come to their senses and want to get back together – that's not the best call.

"If you have some form of unresolved romantic feelings toward them, it's never going to be appropriate," Raquel Martin, a licensed clinical psychologist, previously told USA TODAY.

Ditto if it's about control: "It can be harmful if one party is hoping to use friendship to reinsert themselves into the other's life, or to destroy future romantic relationships," Fisher says.

Even if romantic feelings aren't involved, you may still feel territorial about whom your ex dates.

Keep in mind that your current partner may also feel uncomfortable if you maintain a relationship with your ex. Thankfully, it doesn't seem like Swift and Lautner have that problem, as his wife is a Swiftie.

How to set relationship boundaries.'I am not comfortable sexting,' 'One glass of wine is my limit.'

What to consider before staying friends with your ex

• Boundaries. Don't want to hear about your ex's life, like whom they're dating? Probably a sign to keep your distance.
• Build in some transition time. That doesn't mean a few weeks. Try months. "You're used to having a certain level of support or that extra person to talk to or that first person to text, and you don't have them anymore – that's going to come with an adjustment," Martin says.
• You can be "friendly" without being "friends." "If you both decide to be friends, both of you should be able to respect the new boundaries and expectations of the relationship and be able to allow each other to flourish," Ahrens says.
• Don't hang on to the past. "You can't build a relationship off nostalgia," Martin says.
• Be honest with your current partner. "Showing a new partner that there is nothing deceptive or hidden will help allow for them to feel more at ease," Fisher says.
• Consider your ex's motivations. Maybe they want to keep you on the back burner "but they also want to date other people," Martin says, "and they're still making it so that you're not able to be emotionally available for other people."
• There are other fish in the sea. Just because the relationship was good doesn't mean you won't find that elsewhere.

Generally, though, check in with yourself. "If you want to be friends with an ex, it's important to first be honest with yourself about your motivations and feelings," Nadeau says. "If you find that being friends with your ex is causing more pain than joy, it may be time to reevaluate the friendship and consider ending it."

It seems you'd have to pry Lautner's friendship bracelets away with a crow bar.