how is the kamon passed down?

Does the kamon go from the mother to the daughter? When the girl get married does she take her husband's kamon or does she have her father's or mother's kamon?

Mon are associated with names. So when a girl marries, if she takes her husband's name, presumably the name will have a mon associated with it, and that's her new one. If her husband takes HER family name, she keeps the name and the same mon.

But in this day and age, it is a bit more flexible. Since mon aren't official, some people adopt one they like, or they just use whatever is available (for example, when renting wedding kimono or funeral kimono). And some names don't really have mon necessarily, at least not historically.

I agree with the first half of scarreddragon's answer. But there also is the optional "onna-mon" (women mon) rule in which a wife can wear the mon her maiden name on her kimono.

But in this day and age, it is a bit more flexible. Since mon aren't official, some people adopt one they like, or they just use whatever is available (for example, when renting wedding kimono or funeral kimono). And some names don't really have mon necessarily, at least not historically.

This is all completely new to me and I was never allowed to use a kamon other than the two that were appropriate for whatever the occasion was, but I'm in my 50s. Of course, if you are non-Japanese or a descendant of a family that immigrated to Japan quite recently, it's natural to not have a kamon at all.

But there also is the optional "onna-mon" (women mon) rule in which a wife can wear the mon her maiden name on her kimono.

Ahh, that explains why at my husband's grandfather's funeral, his Aunt wore our family mon even though she's got a different family name. That's good to know!

Also regarding choosing the mon, for example, recently my husband and I chose our wedding kimono. On his, he asked how they will match the family mon (if they have separate haori with each mon, or do they send them out etc.), and the answer was really surprising. Apparently they keep a small selection of the most common mon, but for the most part if you want your family mon on any of the kimono, haori or kurotomesode, etc, you have to pay extra to have a sort of sticker put on. Turns out, most young people don't care which mon is there, and will wear whatever they're given, even if the mon is incorrect. I guess a lot of people don't know their family mon, or it isn't a big deal, or don't have one... enough that wedding rental places don't really worry about it. Seems like the younger generation doesn't care much.

Of course my husband wants his family mon, so we're having to pay extra. And for my mom's formal kimono, since I'm American, I chose the mon that happens to be the same one used by the city, since it's the reason I'm in Japan and met my husband in the first place!

Ah, that sort of explains it. We used to take it for granted to pay extra for our mon back in the 80s where the trend was to invite your aunts and uncles as well as your bosses to the very proper reception of your wedding, while friends were mostly invited to a more casual party.

Being naive I suggested that I may use my maiden name's mon on gifts for the guests, and the venue politely suggested that it won't look too good, or actually she said with a smile that I'm simply "not supposed to."

But I recently attended my niece's wedding in which modern technology had made it possible for the bride and groom to wear kimonos on a completely different day just for a photo session. On the day of the banquette, only the proper-looking photos of the kimono were presented at the hall while they wore tuxes and dresses for the day. The photos were conveniently shot so that you won't even notice the mon, and the kimonos were one of those modern modes anyway.

Additionally, the majority of the guests were friends of their own very young generation, so it made sense to save money and focus on the friends' dancing attractions rather than the proper traditions. Due to the system of this modern venue, parents had very little to do with anything, and no mon was printed on gifts.

So my opinion is that it's not that you can easily use random mon, but that now there seems to be more ways to get away with not properly showing/discussing your mon.

More back to the topic, when relatives get together, the topic of kamon often does become a major conversation theme. So it's still culturally a sort of a big deal today. But then, I keep forgeting what my kamon is!