Borders allow movement between the planes in a smooth, almost uneventful manner. Travelers may not be aware of the boundary and slip unaware over a planar border into a new dimension. Such borders may be patches of darkness, banks of fog, or driving rainstorms. Sometimes one plane gives way to another plane so gradually that it’s impossible to tell where the border region starts and ends. Vision across a border may be limited by the nature of the plane or the border (such as being caught in a fog bank), but it is not inherently impeded.
These have a set boundary between one plane and the next, such as a cliff in one plane hanging over a chasm in another plane, or farmland that suddenly gives way to jungle. Magic does not cross firm borders; nothing on the far side of the border can be targeted by a spell unless that spell specifically affects targets on other planes. Soft borders have less determinate edges, and often exist where similar portions of each plane connect. This forms a buffer zone or “quasi-plane” that belongs to both planes yet has an identity and planar traits of its own. Magic crosses the buffer zone into each of the planes. Soft borders normally exist only when there is similar terrain on both sides, so a plane of continual darkness would only have a soft border in situations where the other plane is also under a dark shroud. Shifting borders are the most perilous to travelers because they move back and forth. Similar to the tidal zone of the seashore, sometimes the border area belongs to one plane, sometimes to the other. Magic treats a shifting border as a firm border, but there is a risk that the border will shift, stranding the traveler on the wrong side of the border and in a dangerous plane.
See also: Rift