A boundary is a statement about what you will do, not a demand about what someone else must do. That distinction is the whole thing, and it's where most attempts go wrong — a boundary that depends on another person's compliance isn't a boundary, it's a request.
Why family is the hardest place
With friends and colleagues you choose the relationship and can adjust it. With family you inherited it, the roles were set before you had a say, and the people involved have decades of practice with the old arrangement.
There's also no clean exit for most people. Cutting contact is sometimes necessary, but it's a serious step with serious costs, and most people want something less drastic — which is what boundaries are for.
What a boundary actually is
It names what you'll do, and you're the only person required to do it.
"I'm not going to discuss my weight. If it comes up I'll change the subject." — a boundary.
"You need to stop commenting on my weight." — a request, and one you can't enforce.
The first works even if the other person doesn't cooperate. The second depends entirely on them.
Expect it to go badly first
In a system that's been stable for years, one person changing produces pushback. Often more than the change seems to warrant.
This isn't evidence you've done something wrong. It's what happens when a long-standing arrangement is altered, and it usually settles if the boundary is held consistently. The first three or four times are the hardest.
Being consistent matters more than being firm
A boundary applied sometimes teaches everyone that it's negotiable, which is worse than not setting one. It's better to choose something small you'll genuinely maintain — leaving at nine, not answering the phone after work, declining one specific topic — than something large you'll abandon under pressure.
You don't have to explain yourself
Most people over-explain, which turns the boundary into a negotiation. "I can't stay for dinner" is complete. Reasons invite counter-arguments.
Where it gets serious
Boundaries assume a relationship that's broadly safe and workable. Where there's abuse, coercion or serious harm, that's a different situation requiring different support — a professional, and sometimes more distance than a boundary can provide.
Why the pattern is worth understanding
Boundaries are easier to hold when you can see what's actually operating — the role you were assigned, the rule nobody states, the loyalty you're acting out of. People often find the practical attempts work better once they've understood the shape of what they're pushing against.