Our "rules" have flexed and loosened over time - aside from the basic tenets of honesty and openness (which I think are crucial to any relationship) our only real rule is "Condoms with anyone other than the three of us."
As a result of some misunderstandings we have a couple of "mock rules" (more like "reasonable requests") - MrS requested "Don't have sex in the bed while I am sleeping in it." and I requested "Don't chase me out of bed to have sex with your ex-girlfriend because you thought I wanted a threesome that you never asked me about." MrS also jokingly requested "Don't have anal sex with Dude if you won't have anal sex with me" (which doesn't actually come into play because anal sex is a complete no-go on my end
Initially, MrS requested to be informed that there was going to be sex happening if there was a chance he could unexpectedly walk into it (we all live in a really small house) because he wanted to be braced for it (or choose to avoid that area). After a few months this was lifted as it no longer bothered him
I do think that it is important to actually spell out what people mean by certain boundaries they have - because sometimes they are not clear about what would bother them, even to themselves. For years I thought that MrS and I were on the same page - we had had many conversations after all. Then when I started misbehaving with Dude it became clear that it was not at all clear where MrS's boundaries were - and because he couldn't articulate them and I was blinded by NRE I managed to convince myself, temporarily, that I was following "the rules". Painful details in my "Journey" blog here - the /Jackassery/ section...
I also wrote a lengthy post about a misunderstanding about boundaries in my "Notebook" blog. To quote myself from that post:
Some things I see as “couple” decisions – things that are decided within the multiple dyads that our V consists of. MrS and I get to decide what is ok within the confines of our relationship. Dude and I get to decide what is ok within the confines of our relationship. Actually, MrS and Dude get to decide what is ok with the confines their relationship as well – as best friends and metamours. Additionally we three together get to decide what is ok within the confines of our Vee - things that we all have agreed to or feel as axiomatic – for instance, keeping all members of the Vee “in the loop” if one of us is developing a romantic/sexual relationship outside of the Vee ...OR condoms with anyone other than the three of us (safer sex/contraception agreement). Underlying all of this – each of us individually gets to decide what is ok for each of us individually. So for a given decision we have to pass through the “filters” of – is it ok for me personally?, is it ok for the relevant dyads?, and is it ok for the Vee as a whole?