Koeler
The Koeler is the mark: a broken circle tattooed at the back of the neck, where the collar sits and where no free person wears anything. Unlike the Sigring it cannot be removed, which is the point of it. A slave without her collar is still marked, and a magistrate settling a dispute looks at the nape rather than the throat.
On manumission the circle is completed rather than struck out. A freed person carries a closed circle for the rest of her life, and anyone who looks can see both that she was a slave and that she is not one now. The completion can be undone: a compound held by trained practitioners clears the newer ink and restores the original broken circle. See Manumission#Re-enslavement.
In the bands of the deep Frontier the broken circle means survivor rather than slave, and some cut three short lines through the gap — neither slave nor free, a third thing. See Frizdoeen#The mark.