A leyline is a source of magic that is present or absent in different parts of Mersia.

A leyline consists of three parts. First a source. Next, a stream. Finally, a taper. A source is a single point from which the leyline originates. The source cannot be moved, it is locked rigidly in place. The stream may be moved laterally, but it cannot turn sharply, so in order to move a leyline's path by a significant distance, one must travel along its length and push the leyline gradually in the desired direction.

At the same time a fae comes into the world, a leyline source is created.