Tithes are those children who've been chosen by their families for religious reasons to be unwound. For centuries it was common for Christians to donate a tithe, or one-tenth of their income, to the church. In the society depicted in Unwind, religious folk take that principle a step further by offering up their tenth children to God as a kind of sacrifice.

Lev is a tithe, which sets him apart from the other main characters in the book. Those unwinds who aren't tithes like Lev are referred to, somewhat unflatteringly, as "Terribles." They wear different uniforms from tithes and don't participate in the same activities as them at the harvest camps. Tithes generally look down on Terribles as a lesser species, which in turn generates hatred and resentment from the Terribles.