A planar seam is a location within a stable reality where its metaphysical relationship with another plane is unusually strong. The location remains fully part of its native plane, but the influence of the connected realm may become perceptible because the transition toward that reality can begin with less resistance than it ordinarily would.
The effects of a seam depend upon the plane involved. A seam associated with a Fundamental Realm might alter weather, geology, temperature, currents, or other physical conditions, while a relationship with one of the Reflective Realms may influence emotion, memory, behavior, or the persistence of emotional consequences. These effects do not mean that part of the other plane has physically intruded into the native realm.
Most planar seams do not permit direct travel. They instead provide favorable conditions for a Vergeway to begin or for magic to establish a planar transition. When a seam becomes shallow enough that a traveler can pass directly between stable realities without navigating an extended transition through The Verge, it becomes a Crossing.
Planar seams may be natural, temporary, recurring, deliberately strengthened, or left behind by powerful magical, divine, or planar events. Some endure for thousands of years, while others appear only when particular circumstances briefly strengthen the relationship between two realities.