"It's not based on any letter or anything," Magnus says, a little apologetically. "But name signs don't have to be based on letters or specific signs, and that felt right." He will not be admitting that he realized one day he'd been using it for some amount of time without even being aware he'd given Hanguang-jun a sign! Or that its base sign is dad! Magnus is pretty sure he, himself, is the only person who uses it. Galahad definitely knows what the sign means, though, even though Magnus hasn't specifically told him, but Galahad also knows pretty much everything there is to know about Magnus just from being observant and thoughtful.