Two people fall in love. One grew up in a house where money was discussed at the dinner table and disagreement was a sport. The other grew up in a house where money was never mentioned and conflict was a sin. They marry. They believe they have married each other. They have also married two ledgers of unspoken rules about how a competent adult moves through the world, and those ledgers do not match.
Class inside a couple is not about income at the moment of the wedding. The sociologist Jessi Streib spent years interviewing class-crossing couples for "The Power of the Past" and the finding that organized her book was this: the class you were raised in installs an orientation toward life that survives the move into a shared household and a merged bank account. People raised middle-class tend to be planners. They manage time, money, careers, and feelings the way you manage a portfolio: with deliberate hands, future-oriented, comfortable expressing distress so it can be processed. People raised working-class tend to be go-with-the-flow. They take life as it comes, distrust elaborate planning as a kind of presumption, manage feelings by riding them out rather than narrating them, and treat money as something you respond to rather than something you steer. Neither orientation is wrong. They are not visible to the people who hold them. They feel like personality, like maturity, like just how grown-ups behave. They are not. They are the residue of a childhood.
The First Law of the Manual is Unity, and unity inside a partnership is the work of making one life out of two people who arrived carrying two different operating systems. Class is one of the loudest operating systems and one of the hardest to see, because in the United States especially we are trained to talk about class as money and then to insist money does not matter between people who love each other. Money is the surface. Underneath money is taste, time, language, the texture of childhood holidays, what a vacation is for, whether you call your father by his first name, whether you say "I feel" out loud, whether you save for a roof or for an experience, whether a relative who needs five hundred dollars gets it without a question or after a budget meeting. These are the things that fight when class fights inside a couple, and they almost never fight under their own names.
Bourdieu called the underlying machinery habitus: the durable dispositions installed by your conditions of upbringing, expressed in your body, your speech, your sense of what is comfortable and what is gauche. Habitus is why one partner is mortified by the other ordering wine by the most expensive option and why the other is mortified that this is something to be mortified about. Annette Lareau, studying childhoods, called the middle-class style "concerted cultivation" and the working-class style "the accomplishment of natural growth," and she showed that children raised in each style emerge as adults with measurably different ease in different rooms. A middle-class-raised adult walks into a doctor's office or a school principal's office expecting to negotiate. A working-class-raised adult walks into those same rooms expecting to comply or to be pushed around. Bring those two into a hospital waiting room together when a parent is sick and you will see a fight that looks like it is about logistics and is actually about a lifetime of who was allowed to ask the receptionist a question.
What helps is naming the layer. Not the surface fight about the credit card statement or the holiday plans, but the orientation underneath. You both have one. Neither of you chose it. Both of you are slightly ashamed of parts of yours and slightly defensive of others, and almost all of the time when you are exasperated by your partner over something domestic, it is because their orientation is producing a behavior that yours reads as either lazy, controlling, anxious, or oblivious. The work is not to convert them. Streib found that the couples who lasted were not the ones where one partner won and remade the other in their image. They were the ones where each partner could see the orientation and choose, in specific moments, to flex toward the other's way rather than treat their own as the default. That is the First Law operating in a kitchen at eleven at night when you are tired and the bank statement is open: unity is not sameness. It is two operating systems learning to run on one machine without either pretending the other is not installed.