- Tessa Gray: "Why bother? Why not just steal the human child and leave nothing in its place?"
- Hyacinth: "Why, because that would not be fair. And it would breed suspicion among the mundanes."
- ―Tessa and Hyacinth talking about the changeling practice, Clockwork Prince
It is a practice among faeries that when their bloodline has weakened and thinned, they would take a suitable, healthy human child and replace it with a sickly fey child—with the assurance that the mundane family the child will be left with is just as suitable. This is a way of strengthening and expanding their bloodline because the human child takes on faerie attributes and some magic during his/her time in Faerieland. Though not always the case, should the fey child survive, they may get to live out a mundane life without ever knowing their origins.
The Covenant prevents the Clave from interfering with this practice because they believe that it is preferred over the other way of bringing in fresh strong blood into their lines, which is the luring and abduction of adult mundanes into their revels. There is also no foreseeable better alternative to the process for refreshing the faerie bloodline, and it is justified by the fact that both switched children are at least raised in loving homes.
- The concept of changelings has long existed in folklore and medieval literature, though with varying stories. The term changeling most often refers to the child, which could be the offspring of a fairy, elf or other legendary creature, secretly left in the place of the human child, though the term is also sometimes used to refer to the child who was taken.
- Elizabeth Gray, who was switched with Elizabeth Moore, is, in a way, a changeling.
- Sebastian considers Jace and referred to him as a changeling as opposed to his father's adoptive son.