Each of the tunnels in These Strange Worlds contains thousands or possibly millions of Doors. No two doors are exactly alike in size or appearance. In many cases, the character of the Door gives some hint concerning the reality beyond. In a few cases, there appears to be no connection between the Door and the reality at all.
The Doors are usually about twenty feet apart and never more than a hundred feet apart.
There are two kinds of Doors: Doors in and Doors out. Both are created with a Reality Ripper, or as many call it, a 'Thresholder' or 'Pivot.' These trans-dimensional objects are rare and priceless. Like all other Door-related tools, they can not be manufactured, but must be found (or stolen).
Doors in are created when somebody in a tunnel uses a Pivot to create a passage into a reality. Which kind of reality depends on the tunnel.
Doors out are created from within a reality and always open into the tunnels. Once created, they pretty much act the same. When a new Door out appears, it invariably appears between two existing adjacent Doors. Despite systematic attempts, nobody has ever actually seen this happen.
In Nexus City, the Arena is jointly owned by anyone who possesses a Reality Ripper. By tradition, time in the Arena is allocated based on the number of Reality Zappers owned. These owners are known as the Arena Alumni Association.
There are an estimated 25,000 Reality Zappers, perhaps half belonging to Arena Alumni Association members. No known person or group controls more than about 500 Reality Zappers. Groups known to have between 250 and 500 include CURBIT, the Zone Pioneers, and Emperor Heinlein.
Most doors are locked. You can see through them but you can't enter without a Key. There are all kinds of Keys. When a door is created, one or more keys to that door are also created. There are also 'Master Keys' that can open many doors.
Master Keys open nearly any door (including Doors into a reality and out of a reality). All you have to do is touch one and concentrate.Master keys often have some kind of indicator that points the nearest door.
By default, keys open a door only long enough to allow the person with the key to use. With practice, somebody with a Key can hold it open long enough for several people to pass through.
If a Door closes while somebody or something is part way through, that somebody or something is severed into to pieces, one on each side of the door.
Every new door starts out locked. A latch prevents a key from working. Usually only the person who sets the latch can remove it. Doors can also be opened using 'Crow Bars' or "Lock Picks' that can batter down or force open locked or latched Doors.
A Wedge closes the door forever. You can look through it but never pass. Heinlein has wedged most of the doors in that enter and leave his Empire. He employs the 'Mallets' -- a well trained and equipped team of Zombies to wedge new Doors as soon as they are created.
Reality Zappers and other Door-related tools can take many forms. Some are small and portable, others so large they are nearly impossible to carry. Some have a mutable form and can be forced to reconfigure by magic users or nanotechnology. They can also be mounted into a standard interface (key shaped or hammer-shaped, for instance, or with a computer interface).
The Zone Pioneers famously configure their Zone Zappers as invitations to join and configure Master keys as a glowing arrow imprinted on each member's hand.
New Reality Zappers and other Door-related tools are occasionally found. Magic, science, or psionics can sometimes detect Trans-Dimensional objects and when a new reality is available, "Zone Prospectors" often rush in to locate and secure new Zone Zappers, Door-related tools, or other powerful trans-dimensional objects.
In the Yellow Strange Worlds, specific objects may be echoed through many realities.For instance, the American Liberty Bell is known to be a Wedge. Any Yellow era from about 1790 to the present era is likely to contain an echo of the Liberty Bell/Wedge.
Heinlein's Empire has an insatiable need for wedges, so one of his teams of magical Zone Prospectors is sure to try to steal the Liberty Bell from a newly-opened reality. The USAs invariably oppose this desecration of one of their most powerful symbols of freedom. On numerous occasions, teams of magicians or magical monsters have fought it out with USAs commandos. The residents of Philadelphia seldom know what hits them.