A planar portal is a link between any two planes of existence or between two points within the same plane. Portals cannot exist, however, in the Flow or between points within separate crystal spheres in the Material Plane.
Both Sigil and Bleak Haven are known for housing a great number of portals to destinations across the multiverse.
Portals vary in terms of their origin, form, consistency, and method of entry. The portals of Sigil and Bleak Haven are typically spontaneous, framed, temporary, and require keys.
Spontaneous portals pop into existence naturally, without being summoned by a particular entity. They can be permanent or temporary, and are often unpredictable in terms of where they lead and how they can be entered.
Summoned portals are created by magic items, spells, or rituals. They are often open for only a brief moment and lead to a specific destination chosen by the creator.
Framed portals must be bounded along their entire edge by solid matter. They tend to appear in doorways, windows, and the like, and they are usually around 1-10 feet in diameter. If the frame of the portal is broken or relocated, the portal disappears. Most spontaneous portals are of the framed type.
Freestanding portals can exist as if suspended in mid-air. They may even wander about instead of staying at a fixed point. Most summoned portals are of the freestanding type.
Permanent portals remain continuously available. They are not necessarily always open or visible, but a permanent portal appears and opens in the same way whenever the entry conditions (if any) are met.
Temporary portals are not continuously available. They are accessible for a limited period, after which they become inaccessible for a length of time, or even perhaps never open again.
Restricted portals only open when certain specific conditions are met.
Unrestricted portals can be entered at any time, requiring no special conditions other than for the creature or object to pass through the portal's space.