Not every portal requires a gate key to operate, but many do. To use such a portal, a traveller needs to have the gate key when he or she steps through. Many gate keys allow their portal to remain open for a short time, which lets multiple travellers pass through with the use of a single gate key.
Gate keys tend to be objects or phrases that are neither too common nor too rare. They aren’t so
common as to cause accidental use of a given portal, nor are they so rare that a portal can never be used. (Although both such portals can exist somewhere in Sigil, and these alien lead to the wildest adventures)
Gate keys. once established, never change. A gate key usually has a connection to its portal’s
destination. providing a clue to where a portal opens up to when the path isn’t widely known. Numerous proprietors throughout Sigil claim to keep logs marking portals, their destinations, and the gate keys required to operate them (if any), and for a price they are usually willing to provide such information to those who ask. Check out Tivvum’s Antiquities in the Market Ward when you’re in need of a gate key.