”In general, perpendicularities are created by the presence of a Shard on the planet. The concentration of so much Investiture on the Cognitive and Physical Realms creates points of ... friction, where a kind of tunneling exists. At these points, Physical matter, Cognitive thought, and Spiritual essence become one—and a being can slide between Realms.
Under certain circumstances, the Realms will bend to the point where they are no longer separate and are instead connected by a perpendicularity. The most common cause of a perpendicularity is a high concentration of Investiture in either the Physical or Cognitive Realms—though this is harder to achieve in the Cognitive Realm—but other methods work as well. Because they are built out of Investiture, a perpendicularity would likely collapse if Nightblood were wielded in its vicinity for a significant period of time.