"Harmonics are an invisible feature of places," Promise says. "They vary, usually continuously but not always and at various rates. Sort of like... ground level. Except in three dimensions. They're one of the things that matters for sorcery, and when they are all the same it's a lot easier to take them into account. I don't know why mortals wouldn't already have sorcery if it's flat everywhere. Maybe it's only flat some places."