This is the private signal flag that I fly from my boat (or anywhere, really). A private signal (or a house flag) was traditionally a flag that represented the owner of the boat. They aren't very common anymore—the closest I usually see is someone flying Blackbeard's flags —but given my interest in archaic communication systems, here we are...
As best as I can describe it, it's a blue swallowtail flag, charged with two green right triangles, each surmounted by a white chevron, the peak of the first at the first third and the peak of the second at the crutch, with the second overlapping the first.
I designed it near the end of the summer of 2021. This was the favorite of a half-dozen or so iterations. All of them were blue, green, and white and vaguely represented both mountains and the letter M. All of those are fairly common elements, but I wanted the flag to be somewhat unique. After searching through Flags of the World, Wikipedia, and reverse image search, the most similar flags I could find were Ed Mitchell's reimagined flag for Montana and the flag for the city of Seward, Alaska. The swallowtail shape helps set it apart from most flags. It fits with the theme, since private signals were traditionally swallowtailed (perhaps for the same reason...). Along the way I also found the FlagWaver web app, which was really helpful to visualize how the flag would look with and without wind.
If, somehow, you found this page after seeing the flag, do let me know.