WASHINGTON — My best guy friend is sitting across from me as I type this, playing footsie with me under the table. We've been friends for 10 years, since college. We can talk for hours about things big and small; we can also sit comfortably in silence. He makes me laugh, always, but has sincere words when I need a lift.

It's the perfect relationship. Except, of course, for when I go home, try to mentally decode the meaning of footsie and then turn to my roommate or my sister or anyone who'll listen and say, "UGH! WE'RE SO PERFECT TOGETHER, WHY AREN'T WE DATING?!" And other sane things like that.

Pop culture abounds with examples of friends who've navigated (or attempted to navigate) the path to romance. Think "Friends," in which Monica and Chandler get together. And "Little Women," when Laurie longs for childhood pal Jo March. And "When Harry Met Sally," which explores the muddy waters of sexual tension to determine if, in fact, men and women can be friends.

Can men and women be friends? I mean, can they really be just friends? OK, yeah. And yet:

"All friendships, even same-sex ones, have ambiguous and changing boundaries," says clinical psychologist Linda Sapadin, author of "Now I Get It! Totally Sensational Advice for Living and Loving" (Outskirts Press, 2006). "You may think somebody's a best friend, and they just consider you a casual friend. How it's perceived is not always the same."

In other words: Your perspective can shift. Suddenly you see a friend as desirable, but he or she still sees you as only a friend. Which leaves you with two choices, Sapadin says: You can try to change it to a romantic relationship. Or you can learn to live with it so that there's flirtatious banter — footsie, anyone? — but nothing else.

It's sexual attraction without acting on it. And the primary reason many of us don't act is fear: the worry that if our friend rebuffs us or the move from platonic to romantic fails, the friendship is irrecoverable.

Such was the outcome for Amy Ewen, who was close friends with her co-worker Peter until they enjoyed a whirlwind romance just before Peter left to spend a year traveling in Asia.

"I was so happy, but it was really bittersweet because he was leaving," Ewen says. She wasn't expecting them to stay together long-distance, she says, but they split with the assumption that there would be something on the other side: a continuation of their friendship.

Ewen left her job to travel, too. When she returned from New Zealand, where she'd met someone else, Peter was back, and she wanted to reconnect as a friend. He never returned her phone calls.

When she finally ran into him one evening, he was standoffish. He shook her hand as though they were business acquaintances and then blurted out that he wasn't in love with her.

"I was remembering how things were when we were good friends," Ewen says. "He thought I was thinking about being his girlfriend. It's sort of a shame because we got along so well."

It is a shame — but there's a comfort to friendship that often gets destroyed when romantic feelings are raised, an awkwardness that accompanies the transition into, and out of, these feelings.

"It feels very uncomfortable when somebody likes you more than you like them," says Ellen Sue Stern, a relationship expert who has written advice books. Making the transition is "always a risk. You should be really sure you want to take that risk before you make that move."

You know a friend well — the opposite of romances in fairy tales. This prince, he's not a stranger. As for Cinderella? Forget the glass slipper. You've watched her clip her toenails.

Kathy Werking interviewed dozens of opposite-sex friends when researching her book "We're Just Good Friends: Women and Men in Nonromantic Relationships" (Guilford Press, 1997). Many reported that, when looking for a romantic partner, they sought someone with an air of mystery.

"There's a lot of fantasy involved when we meet someone," Werking says. "We create a fantasy about what our lives will be together and what this person is all about. It's not as exciting to be around a person who knows you thoroughly."

When you're single and meet someone new, you size them up to determine whether they're dateable. Call it a superficial calculation, but it's nonetheless deliberate. Friend romance, by contrast, seems almost Freudian.

Take Lynne and Kwame DeRoche of suburban Herndon, Va., now married seven years. When they met, they say, they never considered each other as anything but a buddy. Then came the slip. They were talking on the phone, planning to meet up that evening, when Lynne concluded the call by saying, "OK, love ya. Bye."

Kwame didn't skip a beat. "Love you, too. Bye."

Neither acted on those words until a happy hour three months later when, fueled by booze and perhaps pent-up emotion, Lynne kissed him. That act wasn't so much a matter of crossing the boundary between friendship and romance as erasing it.

"Everything that had happened before that was us dating," Kwame says. "We'd basically been dating for six months and didn't know it."

The right timing often is paired with the maturity to understand the difference between what makes friends compatible and what makes romantic partners compatible. Do you both want kids? Where do you both want to live?

These kinds of talks, so pragmatic and seemingly unromantic, are imperative to saving a relationship.

"The friend definition is very different from how we define our romantic relationships," Werking says. "We have different expectations. Flaws that are OK in a friendship may not be OK in a romantic relationship."

But if the flaws are benign and the spark is there, well, that's a great place to be. After all, Stern says, "the healthiest relationships are those that are maximum safety and maximum passion."