It's an issue that divides people of all ages and genders, both introverts and extroverts: When, if ever, is it okay to talk on a plane?

If planes had an equivalent to the quiet car on trains, we wouldn't be here. But in the air, there are no such parameters keeping us on the same page - save the law banning passengers from making calls on U.S. flights (thank God). Many people would appreciate a plane-wide mute button, but others welcome the opportunity to connect with their travel companion or seatmate.

"I think some of the nicest conversations can actually arise with fellow travelers," says Thomas P. Farley, an etiquette expert, speaker and author. "I have people in my life who I have met on airplanes a decade or two ago who remain friends."

And so we're left with a nation divided, the in-flight chatters and those who loathe them. The only way to keep things civil is to establish some ground rules. Some of them already exist unspoken, but just in case, we'll spell them out.

"My definition of etiquette is being mindful of how your behavior affects other people," says Jacqueline Whitmore, a former flight attendant who wrote a book on workplaces etiquette. "And when your behavior affects other people negatively, then we have a problem."

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Acknowledge the crew

When you board your flight, you may notice workers wearing uniforms: flight attendants, pilots, gate agents, aircraft cleaners and bag handlers. Those are human beings, remember, and they make your flight possible. The bare minimum is to acknowledge their existence.

You can keep it simple: eye contact, hello, goodbye and, especially, thank you. "Just be kind," Whitmore says. "That doesn't take a whole lot of effort and it costs you absolutely nothing."

It shouldn't be the only reason you're being decent, but Whitmore says you might be rewarded for being polite and patient with flight attendants. From a free drink coupon to a seat upgrade, "you'd be surprised to how many favors you can gain," she says. "When you're kind you do stand out."

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Greet your seatmates

If your seatmate is a stranger, they're still a stranger you'll be sitting with shoulder-to-shoulder the entire flight, pressed together like a sentient panini. It's bizarre not to say hello in such intimate quarters.

Go ahead and soften the tension with a quick hello. Anything additional is optional.

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Look for cues that strangers want to chat

Should you be feeling social - it's fare game to engage with your neighbor at the beginning and end of your flight. Plenty of travelers are down to chat, and plenty more think doing so is akin to committing a crime.

Look for signs to distinguish one group from the other. They can be obvious: headphones in, eye mask on, a T-shirt that says "don't talk to me."

But it's not always that straightforward. If you're keen to talk, put the ball in their court. Test the waters with an innocuous opening line or two. Try something like, "Headed home?" Or "I still can't believe we're on a 16 hour flight to nowhere." Then see how it lands. If the person answers with a thin smile and a nod, or a two-word response, drop it.

"It's a shame not to at least try," Farley says. Even if your conversation doesn't go further than a little small talk, "you've humanized a very dehumanizing experience which is today's air travel."

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Keep your voice down, unless you're putting in a drink order

A plane is not a private place. It's shared, similar to a doctor's office or a museum. Keep that in mind when talking, and use a hushed voice over a booming one to respect the travelers around you.

"One of the most annoying things is people who talk loudly, whether it be on the phone or to their seatmate," Whitmore says. "Some people want to rest, some people want to get some work done . . . and when you've got two or three loud people sitting behind you, it's very disruptive."

Whitmore has gone so far as to switch seats to avoid loud talkers on flights. But if you can't get away, "sometimes you have no other choice but to politely turn around and say, 'Would you mind lowering your voice? I'd like to get some work done,' or 'I'm trying to sleep,'" she says.

There is an exception: "Speak up when you're dealing with a flight attendant," Farley says.

Throughout the trip, "Flight attendants constantly have to say, 'I'm sorry, could you speak up? Could you say that again?' So speak clearly. Don't mumble. Enunciate . . . you're down below and they're up high."

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Limit conversation to your immediate neighbors

To keep conversation quiet, engage with your immediate neighbors only. When you start shouting across rows, "you start to hold the rest of the plan hostage to your conversation," Farley says.

If you are one of those nightmare pairs who book the aisle and window seat, leaving a stranger stuck in the middle, for the love of all things holy please do not talk over them. Yes, this happens - they have even been known to pass snacks back and forth over the middle seat.

With fights over the arm rest and the discomfort of being sandwiched, it's uncomfortable enough being the middle seat passenger. "But now if you've got two people on either side of you engaged in full-on conversation, I just I can't think of a worse fate," Farley says.

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Keep it PG

When it comes to what to talk about, Whitmore has a good rule of thumb. "Don't share anything that you wouldn't want posted on the front page of The Washington Post," she says.

What you're comfortable sharing is up to interpretation, but at the very least, limit explicit content, from topics to profanity. It's not the end of the world for other adults to hear your dirty secrets, but it's a courtesy to keep them to yourself.

"Even if there aren't children around, some people are just totally offended by profanity," Whitmore says. "It's just not appropriate - like watching an X-rated video in front of a family of four."

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Make calls quick, quiet and essential

As the age of unplugging comes to an end, it's easier than ever to make and receive calls while you travel. While you're not allowed to talk on the phone in-flight, you may be tempted to pick up a call before or after takeoff.

If you must answer it (maybe you're expecting news from a doctor, or a loved one needs help), "avoid cell yell," Whitmore says. "Keep your voice to a conversational tone . . . and keep the call brief."

And please, no speaker phone. Ever.

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No talking on a red eye

A red eye is torture, usually booked because it can save you time or money but never worth its physical and emotional toll. Red-eye passengers are fighting to get the worst sleep of their lives, don't make it harder on them by talking.