As a professional jazz pianist, Tim Clausen knows a lot about making beautiful music on his own. But in writing his new book, “Love Together: Longtime Male Couples on Healthy Intimacy and Communication,” he learned more than ever about what it takes for two people to create long-lasting harmony.

“Love Together” is the culmination of three years of work and over 100 interviews that Tim conducted with long-term gay couples from across the country. Though he’s not in a relationship himself, he found himself drawn to studying what makes successful, long-term gay couples tick: What are their joys? What are their challenges? What are their secrets? Using the interviewing skills he honed in work as a jazz historian, Tim contacted couples across the country to share their very different stories.

And indeed, no two are the same. From a Christian music duo to the Canadian makers of erotic comics, these couples come from all walks of life. Some of the men have been together for 15 years, others as long as 60 years. In fact, Tim closes “Love Together” with the poignant tale of a couple that had been together for nearly six decades; one of the men passed away shortly after their joint interview, so Tim returned for three separate follow-up chats with the widower to discuss the experience of living on without his longtime love. Their final interview took place on what would have been the couple’s 61st anniversary.

Some of the couples are unmarried, while others have tied the knot: America’s first same-sex military couple to marry is among Tim’s diverse interviewees. And Tim found that the act of marriage really did have an emotional impact even on couples that had already been together for many years. “It really did something psychological for them,” says Tim. “One participant told me, ‘Prior to marriage, it was like we were on shifting sand. Now we’re on very solid ground.’”

And though many of the couples are childless, a significant number (about 20 percent, by Tim’s estimation) are also dads.

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So is Tim, incidentally. He has a 26 year-old son from a past relationship with a woman, and for many years struggled with being a long-distance dad. So back in 1995 he founded the Milwaukee Gay Fathers Group, which he led for 10 years. Two of the men he interviewed for “Loving Together” actually met through the group. And Tim noticed that even for longtime couples, there is a new and unique bond that arises when they become parents together.

“Some of the men told me that seeing a partner as a dad made them love him even more,” says Tim. The dads ranged from those who fathered children independently in past relationships, to those who became parents together via surrogacy. One couple even adopted a baby they found abandoned in a NYC subway. However they arrived at fatherhood, the men found that it was a journey that enhanced their relationship together. “Seeing their partner in that newly expanded role was a really profound experience,” explains Tim. “It added a new dimension of love to their lives.”

Fatherhood also helped couples with communication skills, something that all long-term couples credit as being crucial to their success. “Having open and honest communication about everything is really important,” says Tim, recounting some of the most common advice shared by interviewees. “Anything has to be on the table for discussion.”

That includes discussions of monogamy, says Tim, who found that the issue of sexual exclusivity is one that many of the long-term couples negotiated at some point in their relationship. “In heterosexual relationships, people tend to follow a standard script that includes monogamy,” says Tim. But he found that many decades-spanning gay couples approached the issue in more varied ways: Some started in exclusive relationships, but eventually opened up to include other sexual partners. Other couples gave each other allowance to explore outside the relationship, within certain parameters. And still more opened up their relationship only temporarily, before returning to full monogamy. These conversations aren’t always easy, but that they happen at all helps many long-term gay couples remain “above board” with their desires – which can ultimately makes their bond of trust stronger, not weaker.

“When you’re able to communicate about even the difficult things, it leads to less sneaking around,” says Tim. And although many more traditional types blanche at the idea of open relationships, there’s one lesson that all couples can take away from the concept: In successful relationships, partners allow each other to change and grow. “Long term” does not mean “stagnant.”

“It’s all about accepting a person’s good, bad, and indifferent; the whole package,” says Tim of what he learned from these successful couples. “It’s about taking them for all their assets and all their liabilities. And it’s about allowing them to change. If you’re not ready for that, it’s probably not going to work long-term.”

Visit “Love Together” for more details on the book or to order your own copy.