THE bedroom is off-limits. Butts in the toilet are a no-no. The window must be open. And, in the car, cigarettes stay in the pack. These are smoking rules. They are made by couples after the infatuation of romance wears off and before they start sharing the bills. In the union between a smoker and a non-smoker, these pacts, woven with the rubber bands of compromise, are necessary. What the rules are and how they're made is a personal matter. ''When I was a non-smoker and dated smokers, I was a real bitch,'' says Patricia. ''I'd nag my boyfriends to death with 'I won't kiss you if you smoke,' that kind of thing.'' But the 24-year-old Australian, now a serious smoker, lives with a non-smoker. ''He nags me now and then, but he knows it won't do any good. When he wanted me, he put up with my habits. Now he's got me, he has to live with them.'' That he is a former smoker helps, she adds. ''He understands.'' The rules, such as the window must be open if she smokes, are nothing new. ''I was raised by a mother who is a non-smoker. The only place my father was allowed to smoke was outdoors.'' Smoking was never a problem during the courtship or the first two years of marriage for Mei-cheng Leong and Frederic Grisez. She was a social smoker; he, a seasoned veteran. But when she got pregnant, the travel executive quit. Grisez agreed to smoke on the balcony and never around the baby. Today, their daughter is three years old. ''If my daughter has a cold or the flu, the doctor always asks, is there a smoker around,'' says Leong. Their daughter's presence and her health dictate when and where he will smoke. ''It's a bad example for my daughter, I know,'' he says. ''I've tried to quit. My wife hates my smoking and it is an issue in our marriage.'' But he respects the rules they make. When non-smoking guests come to their home, he will ask if anyone minds. ''Somehow, you just work it out with your partner. But it's tough.'' Brendan McSherry, a non-smoker, works in a smoking environment. He's the manager of a bar/restaurant in Lan Kwai Fong. The woman he lives with smokes four cigarettes a day. ''Since our hours are different, I'm rarely around when she's home. She never smokes in front of me. We agree to that.'' And the bedroom is a smoke-free zone by mutual consent. Nagging is rare. ''If I nag her about smoking, she nags me to quit drinking. So we each have something to change.'' Passive smoking is the least of McSherry's worries ''unless someone is blowing it right in my face. In Hongkong, there are many other bad things (threats to health).'' On occasion, a non-smoker will come to John Irvine's smoking cessation classes at Adventist Hospital. They come to understand the addiction or to lend support to a spouse or relative who is trying to quit. How a smoker can live in harmony with a non-smoker depends on compatibility and mutual respect,'' says the director of Adventist's health promotion/wellness department. ''Smoking is a personal decision. It boils down to being accountable for oneself. A supportive environment helps.'' Impending parenthood always brings couples to the classes. ''Bringing a new life into the world is a genuine reason to change.'' But Irvine says the chances of a former smoker returning to smoking after the baby arrives are high, especially if the smoker doesn't have long term solutions in how to deal with stress. ''When something major happens, a divorce or loss of job, they go right back to an old habit.'' Smokers are often the support person when a friend or spouse is trying to quit. By the smoker's support of someone's attempts to quit, he gives permission and respect. ''Many smokers are fiddling around until the right moment arrives,'' adds Irvine. ''Sometimes, if one is courageous enough to quit, it will be the occasion, the catalyst for the other to quit. The dynamics of smoking are complex. And even though smokers are fully aware of the medical dangers, it is not enough to change, according to Dr Melanie Bryan, a clinical psychologist. ''When you feel no immediate side effects, it's hard to experience the danger. But when couples become parents, they want to do their best, so they will alter their ways.'' She said when a smoker quits and their partner continues to smoke, there is often a breach of loyalty. ''You have a little less in common,'' continues Bryan. ''Sometimes it can distance people.'' Bryan believes that the level of commitment to each other determines how the smoker and non-smoker can live together in relative harmony. A birthday was the pivot point for one couple. ''When we began to date, his smoking didn't bother me,'' said Judy, 32, a non-smoker. ''But we've been together now eight years and I never bitched until recently. ''When you get older, you appreciate things more, you want to take care of yourself. And you get set in your ways. ''I hated kissing him with nicotine on his breath. I hated the way my hair smelled. He had always been good about the rules. Windows open, fan on, if he smoked in the kitchen. No smoking in bedroom, the bathroom, no butts in the toilet. When non-smokers come over, he always smokes outside. ''Then one day, out of the blue, he gave it up. It was his 40th birthday.''